Thousands of "Error adding sparse file entry. Could not read metadata ..." warnings

Server: UrBackup 2.5.32 (running on Ubuntu 22.04)
Client: UrBackup Client Backend v2.5.25.0 (running on Ubuntu 22.04)

I noticed that the backup logs contain thousands of these warnings:

Error adding sparse file entry. Could not read metadata from /mnt/...

I wonder if this is anything critical? I am confused because it seems to happen to any file included in the backup. The last log contains over 7k of these. Also, it gives me an ETA of 3 hours for a 28 MB incremental file backup (I guess this is only the amount of actual files to be transferred, but still all files need to be checked, whether their hashes still match, but 3h still seem to be a bit too long for that).

Maybe this is a file permission or filesystem issue? I am using a Hetzner Storage Box for the backup files and mounted it via CIFS.

Any ideas?