iPhone shows unknown Artist or Album

I use the grouping field in iTunes in order to be able to add the language of a track. For some reason some(!) albums on my iPhone suddenly got messed up. Instead of showing the titles, some(!) tracks are displayed with the information of the grouping field. Plus, those tracks are "grouped" within the album view by showing the language and "by unknown" above them. When I play the track, all the information is shown in the "currently playing view".

In my iTunes library however everything is displayed perfectly fine. Also, all my tracks are tagged with both the interpret and the album interpret. Compilation field is unchecked as well. Really disturbing about this whole thing is, that deleting the information of the grouping field doesn't have any effect on the described problem.

I tried finding out the difference between the tracks that are displayed fine and those that aren't, but all fields contain the same information. One thing that sometimes messed with my library was whenever I edited tags while iTunes was syncing with the cloud. However, in those cases the information (mostly artists appearing twice) were both displayed wrong in iTunes and on my phone.

Any suggestions how to fix this mess, anyone?