Lightroom 7.0 or later is now required to sync files with the Creative Cloud.
Reverting to CC2015.x will cause a loss of sync capabilities. My best suggestion is to see if we can isolate the cause of your crashing when your have the Lightroom Classic CC version running.
Did you uninstall Lightroom Classic CC or is it still on your system?
After the Uninstall/Restart/Reinstall, does the crashing still occur?
[Solved] or at least it worked for me...
I had similar problems twice recently with Lightroom. The first time was shortly after the upgrade to version 8.0, September? October 2018? The symptoms were simple: start Lightroom, and within 60 seconds, crash report dialog.
I tried various ways to fix this, the details of which I forget, but in the end, I got out of the problem by doing a Windows system restore. I was fortunate enough to have a restore point that was just a few days old. After that all was fine.
The second time I had this problem was last night. Suddenly Lightroom was very slow to load and then started crashing again. So I took a different approach.
My suspicion was that there was something wrong with the catalogue itself. I therefore deleted the catalogue file (SomeName.rlcat), and the two files that Lightroom creates named after the catalogue file:
And the folders
Then I restored the catalogue file SomeName.rlcat from my last backup and restarted Lightroom. That solved the problem, at the cost of throwing away the image previews.
Thankfully I have Lightroom set up to do a catalogue backup every day.
Hope this helps someone else.