Cannon read or delete mail subfolder! Corrupted? How to diagnose/fix?

[Fred5711]

Since a few weeks I get errors in iOS mail when querying (IMAP) Mail Station (Roundcube) on my Synology NAS. To the point that it stopped working completely today. On other devices/apps (Outlook, Windows Mail...) it still works perfectly, except 1 mail-subfolder cannot be read from all places. Seems to be corrupt (not sure). Suspect iOS stumbles over that (not sure). Tried to delete that folder => fails. Now I am stuck.
Has anybody seen something similar? How could I diagnose this situation? How to fix this? If corrupted (database?) then is there a way to 're-index' somehow without losing mails in the folders? Help!
Thanks!!!

Note: I am a bit computer savvy but not experienced with DMS commandline or alike, e.g. if I need to go to 'cPanel' (??read about it) or DMS commandline, how do I do that?

[JohnDoh]

Roundcube is a web based IMAP client similar iOS mail or Outlook only those are software based. From your description it sounds like there is a problem on the server so you need to contact the people who run that.