Konstantin
I'm a long time Parallels user, going back to around '08 with Parallels 4 or 5. Up to now, I've never had a complaint about it that was no resolved in a short matter of time.
Around June or July of this year, I got a chance to try the Beta El Capitan version, which I used with my existing version of Par10/Win7. Worked great for maybe 6 weeks or more or until shortly after installing Win10. I'm guessing maybe a month with Win10.
It now loads up until the Win10 logo appears in the center of the screen with the new "circling" dots. After a few second, the screen would freeze and the only way out of it was to shut the computer down (MBA) and reboot.
I figured I had some conflict between Beta El Capitan/Parallels 10/Windows10. So I didn't use the program until the full retain edition of El Capitan came out. When it did, I installed it, only to find the same problem existed (freezing). Then, the "pitch" came up, touting the new Par 11, which I installed, only to see the SAME problem. I've tried deleting Par11 and reinstalling, but that doesn't correct the problem. I was thinking of going back to Par 10, but I can't find my installation disk (and the problem existed there as well).
Based on my previous history with Parallels always working, I downloaded the paid edition of Par 11 ($50). If I had known that it might not work, I would have definitely tried the free trial version instead. If I can't fix this problem shortly, I'm going to abandon Parallels and go the BootCamp route which I have on another computer that runs Windows 99% of the time. On my MBA, I use or did use Parallels to switch back and forth several times a day. I HATE the BootCamp route that requires a reboot when switching.
Since the problem first surfaced using Beta El Capitan, I would suspect Parallels 10 or 11 doesn't work well with the combination and the fact that Parallels did work VERY will with Win 7 seems to add to that theory. It also seemed to work fine with Beta El Capitan/Win10 until it started freezing on rebooting.
I'm not much for trouble shooting and don't have a clue as to whether I could go back to Yosemite or Win7. Re Parallels 11, I thought about deleting and reinstalling it, but I'm not sure I can remove it entirely. My method is to go into Applications, and drag Parallels to the waste basket and then reinstalling it. Does that remove ALL of the previous Parallels installation?
And if I have to abandon Parallels 11, how do I get my refund of the $50? There seems to be several others on this forum with the same or slight different complaints about Parallels with Windows 10.