Go to the "Settings" area on the web interface and then to "Behavior".

Level 1 should be sufficient for normal use. Please select level 3 when it comes to debugging.

A log of the backup process is recorded in the log file. Please select this when it comes to debugging.

In order to search for an error, you should first find out how you can reliably provoke it.
Before you start this faulty process, please set the log settings as shown above.
Under the log monitor fields you will find a button "Delete log". Please use it to first empty the log file.
Now start the process that is triggering the error. You can then download the log using the "Download" button and, if necessary, make it available to the developers.

At the end of the backup process, a check is made as to whether all USB storage devices involved are still mounted. If not, up to three backup attempts are made before the above-mentioned error message and cancellation occurs.

The working hypothesis is that the power supply of the USB devices cannot be sufficiently guaranteed by the Raspberry pi. This leads to brief failures of the devices and thus to the connection breakdown.

Little-backup-box will attempt to remount "lost" devices and continue the backup. For devices connected via USB, this will usually only work if it is a partition with a UUID identifier. Therefore, the target device should definitely have a UUID. You can see this in the system info.
If the partition does not have a UUID or the device is a camera, the situation is different: The device is usually re-connected and receives a new USB identifier and therefore cannot be reliably recognized. In this case, the retries will fail. However, the device then appears as a new device in the (internal) list of available devices and another attempt is made.

A continuous power supply can be ensured via a powered USB hub. Basically, problems can simply be due to insufficiently designed power supplies. It was even observed that interference frequently occurred with certain USB cables for powering the raspberry pi.

Error 23 is issued by rsync and is unfortunately the message for errors with many different causes.

It can occur due to incorrect file system rights or due to file system corruption. In the latter case, the system could possibly even be repaired using LLB's Web UI tools.

Insufficient storage space on the target drive can also lead to this error message.

Another known problem is special characters and umlauts in file or folder names. If the source and target file systems are not identical or do not use the same character set, rsync may repeatedly see deviations during the comparison and therefore fail. This problem seems to occur regularly between Mac OS and Linux file systems. The solution here is to use the same file system on the target drive as on the source drive. Otherwise, it's definitely not a bad idea to avoid problematic characters in file and folder names.

The web-based update depends on the apache2-webserver. If the webserver stopps, update breaks. If you can access the web-UI, just restart the web-based update. If the web-UI is offline, you can wait for the "Power down after idle-time" and restart the box or you can restart the box by ssh. A hard reset is always a bad idea...