Ciao Luigi,
I finally found out how to solve the problem. Plex tries to use the teletext data as subtitle data, but teletext data over the main Italian channels is not compatible and cannot be used as subtitle data. As a result the stream gets locked and/or corrupted.
The solution to this problem is to stop Plex from automatically loading subtitle tracks. You can achieve this by going to the server options and by using the following settings:
