Thank you for getting back to me so fast. It is indeed a network share, which works just fine with 2.2.3 (without VSS because Catalina). I did a few tests with a blank new repo, from scratch, only a few GB.
First I tested 2.3.0 with the network share as above (NFS v4):
~/Dropbox duplicacy2.3.0 init -encrypt VMacTest /Volumes/BackupV
Enter storage password for /Volumes/BackupV:****************************************************************
The storage '/Volumes/BackupV' has already been initialized
Compression level: 100
Average chunk size: 4194304
Maximum chunk size: 16777216
Minimum chunk size: 1048576
Chunk seed: 897fe215233abffb006661497326c230eb1428bf36070980af629a7530d69a2f
/Users/vlad/Dropbox will be backed up to /Volumes/BackupV with id VMacTest
~/Dropbox duplicacy2.3.0 -log backup -stats -vss
2019-11-27 21:15:05.471 INFO STORAGE_SET Storage set to /Volumes/BackupV
Enter storage password:****************************************************************
2019-11-27 21:15:11.547 INFO BACKUP_START No previous backup found
2019-11-27 21:15:12.075 INFO VSS_DONE Shadow copy created and mounted at /tmp/snp_064817310
2019-11-27 21:15:12.075 INFO BACKUP_INDEXING Indexing /Users/vlad/Dropbox
2019-11-27 21:15:12.075 INFO SNAPSHOT_FILTER Parsing filter file /Users/vlad/Dropbox/.duplicacy/filters
2019-11-27 21:15:12.075 INFO SNAPSHOT_FILTER Loaded 0 include/exclude pattern(s)
2019-11-27 21:15:14.230 ERROR UPLOAD_CHUNK Failed to upload the chunk 2df7b5a5b8932872c6c2b035c7264f8caf0aa14c6bdf5ec39c16341372028e58: sync /Volumes/BackupV/chunks/2d/f7b5a5b8932872c6c2b035c7264f8caf0aa14c6bdf5ec39c16341372028e58.cmjpxoil.tmp: operation not supported
2019-11-27 21:15:14.231 INFO INCOMPLETE_SAVE Incomplete snapshot saved to /Users/vlad/Dropbox/.duplicacy/incomplete
This failed, so I tested a SFTP connection to the same server hosting the nfs share, and this works just fine. However the speed is very very slow compared to the NFS share (4-5MB/s):
~/Dropbox duplicacy2.3.0 init mywork sftp://vlad@192.168.1.11/test
Enter the path of the private key file:/Users/vlad/.ssh/ansible_rsa
The storage 'sftp://vlad@192.168.1.11/test' has already been initialized
Compression level: 100
Average chunk size: 4194304
Maximum chunk size: 16777216
Minimum chunk size: 1048576
Chunk seed: 6475706c6963616379
/Users/vlad/Dropbox will be backed up to sftp://vlad@192.168.1.11/test with id mywork
~/Dropbox duplicacy2.3.0 -log backup -stats -vss
2019-11-27 21:21:54.647 INFO STORAGE_SET Storage set to sftp://vlad@192.168.1.11/test
2019-11-27 21:21:55.545 INFO BACKUP_START No previous backup found
2019-11-27 21:21:55.914 INFO VSS_DONE Shadow copy created and mounted at /tmp/snp_213689641
2019-11-27 21:21:55.914 INFO BACKUP_INDEXING Indexing /Users/vlad/Dropbox
2019-11-27 21:21:55.914 INFO SNAPSHOT_FILTER Parsing filter file /Users/vlad/Dropbox/.duplicacy/filters
2019-11-27 21:21:55.914 INFO SNAPSHOT_FILTER Loaded 0 include/exclude pattern(s)
2019-11-27 21:21:56.552 INFO BACKUP_LIST Listing all chunks
2019-11-27 21:21:57.217 INFO UPLOAD_PROGRESS Uploaded chunk 1 size 1783321, 1.70MB/s 00:27:40 0.0%
2019-11-27 21:21:57.375 INFO UPLOAD_PROGRESS Uploaded chunk 2 size 1403112, 3.04MB/s 00:15:29 0.1%
2019-11-27 21:21:58.713 INFO UPLOAD_PROGRESS Uploaded chunk 3 size 9941349, 6.26MB/s 00:07:30 0.4%
2019-11-27 21:22:01.014 INFO UPLOAD_PROGRESS Uploaded chunk 4 size 16478452, 5.65MB/s 00:08:16 0.9%
2019-11-27 21:22:02.222 INFO UPLOAD_PROGRESS Uploaded chunk 5 size 6997979, 5.82MB/s 00:08:00 1.2%
2019-11-27 21:22:03.711 INFO UPLOAD_PROGRESS Uploaded chunk 6 size 4435348, 5.59MB/s 00:08:19 1.3%
2019-11-27 21:22:03.907 INFO UPLOAD_PROGRESS Uploaded chunk 7 size 1100142, 5.74MB/s 00:08:05 1.4%
2019-11-27 21:22:04.444 INFO UPLOAD_PROGRESS Uploaded chunk 8 size 3280025, 5.41MB/s 00:08:34 1.5%
2019-11-27 21:22:04.943 INFO UPLOAD_PROGRESS Uploaded chunk 9 size 2192313, 5.68MB/s 00:08:10 1.6%
2019-11-27 21:22:05.766 INFO UPLOAD_PROGRESS Uploaded chunk 10 size 3614789, 5.43MB/s 00:08:32 1.7%
2019-11-27 21:22:07.116 INFO UPLOAD_PROGRESS Uploaded chunk 11 size 5937934, 4.96MB/s 00:09:19 1.9
And lastly, I tested again with the old version (2.2.3) just to get a sense of speed and it is much much faster (25-30MB/s):
~/Dropbox duplicacy2.2.3 init -encrypt VMacTest /Volumes/BackupV
Enter storage password for /Volumes/BackupV:****************************************************************
The storage '/Volumes/BackupV' has already been initialized
Compression level: 100
Average chunk size: 4194304
Maximum chunk size: 16777216
Minimum chunk size: 1048576
Chunk seed: 897fe215233abffb006661497326c230eb1428bf36070980af629a7530d69a2f
/Users/vlad/Dropbox will be backed up to /Volumes/BackupV with id VMacTest
✘ ~/Dropbox duplicacy -log backup -stats
2019-11-27 21:25:19.820 INFO STORAGE_SET Storage set to /Volumes/BackupV
2019-11-27 21:25:27.443 INFO BACKUP_START No previous backup found
2019-11-27 21:25:27.443 INFO BACKUP_INDEXING Indexing /Users/vlad/Dropbox
2019-11-27 21:25:27.443 INFO SNAPSHOT_FILTER Parsing filter file /Users/vlad/Dropbox/.duplicacy/filters
2019-11-27 21:25:27.443 INFO SNAPSHOT_FILTER Loaded 0 include/exclude pattern(s)
2019-11-27 21:25:29.288 INFO UPLOAD_PROGRESS Uploaded chunk 1 size 2145302, 1.02MB/s 00:45:59 0.0%
2019-11-27 21:25:29.312 INFO UPLOAD_PROGRESS Skipped chunk 2 size 1212810, 1.60MB/s 00:29:22 0.1%
2019-11-27 21:25:29.328 INFO UPLOAD_PROGRESS Skipped chunk 3 size 2007459, 2.56MB/s 00:18:22 0.1%
2019-11-27 21:25:29.637 INFO UPLOAD_PROGRESS Skipped chunk 4 size 15443875, 9.92MB/s 00:04:43 0.7%
2019-11-27 21:25:29.643 INFO UPLOAD_PROGRESS Skipped chunk 5 size 5817797, 12.70MB/s 00:03:41 0.8%
2019-11-27 21:25:29.701 INFO UPLOAD_PROGRESS Skipped chunk 6 size 4784606, 14.98MB/s 00:03:07 1.0%
2019-11-27 21:25:29.720 INFO UPLOAD_PROGRESS Skipped chunk 7 size 1220246, 15.56MB/s 00:03:00 1.1%
2019-11-27 21:25:29.794 INFO UPLOAD_PROGRESS Skipped chunk 8 size 12561249, 21.55MB/s 00:02:10 1.5%
2019-11-27 21:25:29.799 INFO UPLOAD_PROGRESS Skipped chunk 9 size 1150653, 22.10MB/s 00:02:06 1.5%
2019-11-27 21:25:29.804 INFO UPLOAD_PROGRESS Skipped chunk 10 size 1266680, 22.70MB/s 00:02:03 1.6%
2019-11-27 21:25:29.982 INFO UPLOAD_PROGRESS Skipped chunk 11 size 16777216, 30.70MB/s 00:01:30 2.1%
2019-11-27 21:25:29.998 INFO UPLOAD_PROGRESS Skipped chunk 12 size 2326396, 31.81MB/s 00:01:27 2.2%
2019-11-27 21:25:30.044 INFO UPLOAD_PROGRESS Skipped chunk 13 size 1989702, 21.84MB/s 00:02:07 2.3%
2019-11-27 21:25:30.128 INFO UPLOAD_PROGRESS Skipped chunk 14 size 2426828, 22.61MB/s 00:02:02 2.4%
2019-11-27 21:25:30.179 INFO UPLOAD_PROGRESS Skipped chunk 15 size 1810704, 23.19MB/s 00:01:59 2.4%
2019-11-27 21:25:30.192 INFO UPLOAD_PROGRESS Skipped chunk 16 size 1895979, 23.79MB/s 00:01:56 2.5%
2019-11-27 21:25:32.170 INFO UPLOAD_PROGRESS Uploaded chunk 17 size 10194482, 16.22MB/s 00:02:50 2.8%
2019-11-27 21:25:32.196 INFO UPLOAD_PROGRESS Skipped chunk 18 size 2982936, 16.79MB/s 00:02:44 2.9%
2019-11-27 21:25:32.227 INFO UPLOAD_PROGRESS Skipped chunk 19 size 4684822, 17.68MB/s 00:02:35 3.1%
2019-11-27 21:25:32.276 INFO UPLOAD_PROGRESS Skipped chunk 20 size 3538023, 18.36MB/s 00:02:29 3.2%
2019-11-27 21:25:32.308 INFO UPLOAD_PROGRESS Skipped chunk 21 size 2850091, 18.90MB/s 00:02:25 3.3%
In conclusion, for now I will stick to duplicacy 2.2.3 until 2.3.0 will work on Catalina with a network share.