so if you play as the fatimids and liberate syria as a vassal, your game crashes.

yes

For me it crashes after 6 months of gameplay regardless of which nation I play.

In case you guys don't know, Paradox released an update that limited the amount of tags that can be available in any given game, to essentially a number just above what is available at vanilla 1444. We don't know if this is intentional or not, but it is most certainly the reasons for your crashes. Qwerty and other mod makers are in contact with Paradox to have this resolved. Hopefully it's fixed soon, this bug will pretty much leave any mod that adds tags completely broken.

I hope they will change that because i play this game just because of that mod

Eu4 is good, but this mod is the main reason it is still going.