I received a solution from Kentico support in the end:
I would specifically look for MACRORESOLVER errors.
This problem with missing tab could be caused by macros that have become signed with incorrect hash. In this situation I would recommend to Re-sign macro signatures by navigating to System > Macros > Signatures, checking both check boxes and Update macro signatures.
Ref: http://docs.kentico.com/display/K82/Working+with+macro+signatures#Workingwithmacrosignatures-Re-signingmacros