Linux Client 2.3.3 unable to sync hidden directories

Normal directories are synced without any trouble. But if I choose (after right click for showing hidden files) a hidden file (e.g. .Thunderbird) the folders inside this directories are not shown. If I choose it anyway I get the message ‘Synchronisationsbericht konnte nicht initialisiert werden’’. To mention it again, this only happens with hidden directories, not with not hidden directories. Desktop is Ubuntu 18.4.