Dear Sebastian,
it first raise after update to 4.7.
In 4.7 i deactivated auto filesync - because every morning the permissions went away.
after upgrade to 4.8, everything was fine since i made a scheduled file sync.
i checked all files physically for wrong characters - even deleted all files (moved them to the outside) and only let the skins file in it.
then i tried deleting the file table and put the files physically one after one again to the filemanager, made a sync, and tried to set proper permissions.
then this error message is coming again and again.
Currently only thing for me was playing back the backup of 4.7 and don't make a filesync.
there is one interesting thing. i only have around 500 files in this portal. But every time i made a resync, dnn deleted all file table entries and put in the files with a new id.
So currently my files are numbered in a range of 15000 to something - thats the id in the file table.....
Even the file table contains valid files - a resync will put them in again as a new file, so with a resync the portal is usless becaus of 'missing' images e.g.
Maybe that helps a bit