I don't like to tirade myself, unless my wife has been in my office and tidies my desk - yes it's a mess but I "knew" where everything was...
...anyway, it depends on the definition of "unplayable".
For my own part, trying to play a save in the Spanish leagues, certainly in the lower leagues, I would consider this bug or problem as contributary to making the game unplayable.
I've made seven signings in my game and five of them are classed as ineligable. I only find this out AFTER I've submitted my 22 man squad, in the Liga BBVA Segunda B4 Division.
Yes, I could continue the save, but given these players are signings integral to my hopes of acheivement with my team, it rather defeats the object of continuing really.
Still, if anyone knows if this is something that can be fixed via the editor, then it's worth sharing your knowledge, as I for one, would prefer not to wait until next year before I can enjoy a "working" game in the Spanish leagues.