Volume Backup stops for hours before resuming (over VPN)

Hello!

I am testing urBackup and considering to use it for my clients production environments.

But i have the problem that the full volume backup stops multiple times for hours before continuing with the job. While the backup is not working the urBackup Client on the Windows Machine is in Idle Mode.
The following error message is in the logfile of the server:

2022-01-26 12:55:35: WARNING: Error retrieving last image backup. Doing full image backup instead.
2022-01-26 13:30:38: ERROR: Constructing of filelist of “PW00CAM” failed: no backup dirs. Please add paths to backup on the client (via tray icon) or configure default paths to backup.
2022-01-26 13:30:46: ERROR: Backup had an early error. Deleting partial backup.
2022-01-26 13:30:47: WARNING: Exponential backoff: Waiting at least 5h 20m before next file backup

In the clients log i havent found anything suspicous:

2022-01-26 13:30:35: ClientService cmd: STATUS DETAIL#pw=7vZkH6D671lw14yRJCVyyQdBkRyeoJ
2022-01-26 13:30:36: ClientService cmd: STATUS DETAIL#pw=7vZkH6D671lw14yRJCVyyQdBkRyeoJ
2022-01-26 13:30:36: Started connection to SERVICE_COMMANDS
2022-01-26 13:30:36: ClientService cmd: #IIEfRzPG9tLM0eC7tpXku#2PING RUNNING pc_done=&status_id=6&speed_bpms=0&total_bytes=-1&done_bytes=0&paused_fb=1#token=GGPgWQCdh6ejOERPnTQu
2022-01-26 13:30:37: Started connection to SERVICE_COMMANDS
2022-01-26 13:30:37: ClientService cmd: #IIEfRzPG9tLM0eC7tpXku#3START FULL BACKUP group=0&running_jobs=2&sha=528&with_permissions=1&with_scripts=1&with_orig_path=1&with_sequence=1&with_proper_symlinks=1&status_id=6&async=1#token=GGPgWQCdh6ejOERPnTQu
2022-01-26 13:30:37: Async index 7f726f5607c54f9a07e1e2242b79cb3c
2022-01-26 13:30:37: Removing VSS log data…
2022-01-26 13:30:37: Script list at “C:\Program Files\UrBackup\backup_scripts\list.bat” does not exist. Skipping.
2022-01-26 13:30:37: Deleting files… doing full index…
2022-01-26 13:30:37: rc=0 hasError=true state=0
2022-01-26 13:30:37: SERVICE_COMMANDS finished
2022-01-26 13:30:37: ClientService cmd: STATUS DETAIL#pw=7vZkH6D671lw14yRJCVyyQdBkRyeoJ
2022-01-26 13:30:37: Started connection to SERVICE_COMMANDS
2022-01-26 13:30:37: ClientService cmd: #IIEfRzPG9tLM0eC7tpXku#WAIT FOR INDEX async_id=7f726f5607c54f9a07e1e2242b79cb3c#token=GGPgWQCdh6ejOERPnTQu
2022-01-26 13:30:37: Wait for async index 7f726f5607c54f9a07e1e2242b79cb3c
2022-01-26 13:30:38: ClientService cmd: STATUS DETAIL#pw=7vZkH6D671lw14yRJCVyyQdBkRyeoJ
2022-01-26 13:30:38: ClientService cmd: STATUS#pw=7vZkH6D671lw14yRJCVyyQdBkRyeoJ
2022-01-26 13:30:38: Selected no components to backup
2022-01-26 13:30:38: Deleting backup of changed dirs…
2022-01-26 13:30:38: Script “C:\Program Files\UrBackup\postfileindex.bat” does not exist
2022-01-26 13:30:38: Async index 7f726f5607c54f9a07e1e2242b79cb3c finished with “no backup dirs”
2022-01-26 13:30:38: rc=0 hasError=true state=0
2022-01-26 13:30:38: SERVICE_COMMANDS finished
2022-01-26 13:30:39: ClientService cmd: STATUS DETAIL#pw=7vZkH6D671lw14yRJCVyyQdBkRyeoJ

I have no file backups configured, only volume backups.
The urBackup Server is running on a Ubuntu 20.04 with btrfs file system. The server itself is connected to one of my clients site over a pptp vpn connection. Client to back up is therefore on the remote site. The internet mode is activated and configured according to the Administration Manual. I have disabled compressed and encrypted transfer of the data. All other settings are default.

Is there any way to edit the Exponential backoff timeout settings or disable it?

Thank you very much!

What version OS of the client?
What UrBackup version? (Client and Server)
Is the VPN tunnel running the whole time?

-ASB