Default product combination not saved after upgrade from 1.6 to 1.7

[myhappyidea]

Hi!

We had a site in PS 1.6, which we upgraded to 1.7 (I don't remember which version exactly, but we have upgraded now to the latest PS - 1.7.6.1). I have tried to look for a similar problem on the forum but I haven´t found this error exactly, maybe its a combination of many things. I have seen that upgrades with combination products sometimes give problems, though.

The thing is, we had tons of products imported. Initially they had no  PS combos, but when we migrated to 1.7 we added them attributes/combinations.  We did have a module before (That did not work under 1.7), that kind of made combinations (my clients product is eyewear and it has many permutations, and sometimes they are not related, I guess thats why they bought it the first time around), this time we have combinations + an extra module for an added attribute that does not impact on the first combinations (does that make sense?)

Well... the imported products don't have a default combo. And we can't even save it. PS does not give any error, and shows us the OK with the green alert on the screen, but it really isn't saving.

What can it be? Some people have had problems saving with old products and the product had tags with weird chars, but we dont use tags... 

Thanks in advance!

Ana