Field Marshal Steel says:
I removed all techs from Britain in their inc file and released everyone.
No crash!
So we must sort out the inc files of Britain, France, Germany, Austria to weed out these invalid techs.
Notice that 3/4 of these countries are in "tier 1" in our tech assignment.
Most common reason for this is an invalid tech ID. Try removing all techapps and techlevels from the INC file for one of those countries, then start the game and try releasing a vassal while playing that country. If you don't get a CTD you will have confirmed that it's an invalid tech ID and should then audit all the entries in the INC files.
I removed all techs from Britain in their inc file and released everyone.
No crash!
So we must sort out the inc files of Britain, France, Germany, Austria to weed out these invalid techs.
Notice that 3/4 of these countries are in "tier 1" in our tech assignment.