No problem with access to the data folder. It is there and a restart of computer and/or program does not help. Something got corrupted in the data directory.
It isn't a question of "access to the data folder", but a question of whether bitcoin is pointed to the data folder. Perhaps that is just a translation difference, but if bitcoin isn't pointed to the right place (e.g. datadir showing the correct path) you might see this message. ;-)
If you are running the data on the QNAP, you would want to check and make sure that the datadir parameter is pointing there vs the standard Windows location.
Usually the data directory is as specified here:
https://en.bitcoin.it/wiki/Data_directoryBut it would be different since you are storing the data directory on the NAS.