Some folders not synced - Strange exclude list

I have a folder β€œflac” which is synced. In this folder there are subfolders from β€œA” to β€œZ”. All of them get synchronized. Except for β€œR” and β€œS”.
The Nextcloud client shows this message (in german):

Why do they not get synced? And why have they been deleted locally?

I never made any changes to the exclude list, so it’s the original one.