3.6 changed the requirement of Clone Vats from an Ascension Perk to a Tradition.
If by this you mean that you started the game on 3.5 and then didn't stay on 3.5... look, I'm sorry, at least read the post announcing new versions (if you can't be bothered to read the actual patch notes, which will make incompatibilities pretty obvious). It is almost never the case that either major or minor version differences (in the sense of major.minor.patch versioning) are straightforwardly save-compatible, and the devs note it on the rare occasion that they are. Patch versions are almost always compatible within their minor version (although even there you might encounter glitches, so read the announcement anyhow).
This is a fair assessment. The comment below yours is excessively harsh imho. I don't live-breathe-die Stellaris. Its a fun distraction when I have some free time. And yeah, I've been burned on games flipping across lines going back to the first patches on 1.0. I come and go with playing this game. Most of the time in the past, the save games are simply listed as "unusable" and so, I knew I had to move on. The changes from 3.5 to 3.6 seem significant enough to have them not work at all. But, it loaded and i didn't notice.....so, I asked a dumbish question.
