See last post in this thread for the fix.
 
I ran into problems with X3 64 bit in 8.1  I had originally installed in it Windows 8, and it appeared to install OK and was working.  I installed the X3b patch but ran into a problem and had to abort it.   I didn't try it again as Windows 7 is my "production" system.  I had a problem with some plugins with Win 7 and X3b so I decided to try it in Windows 8.1.  When I tried to start X3b it would not start and I had an error (don't remember what it was).  I decided to try reinstalling the X3b patch again but it aborted with an error.  I decided to reinstall X3a 64 bit in Win 8.1 but it failed with Dll OCX Regsvr32.exe Exit Code 0x3.  I did a search on that and found several references to that including several for Sonar but nothing that I found was a fix.  I uninstalled X3 and restarted the PC and tried a complete new install of X3a 64 bit in Windows 8.1 but it again failed with the Regsvr32.exe error.  I then tried X2 64 bit as it had previously worked OK in Win 8 (not 8.1) and it too failed to install with the same error.  Finally, I tried an X3a install of the 32 bit version and it installed OK, and the X3b patch also installed OK (32 bit). 
 
During my attempts to install X3a, I also tried it in Win 8 compatibility mode and that resulted in the same error failure. 
 
As it stands now, X3b is installed as 32 bit version and working, but can't install the 64bit version.

I posted this in the other thread also:
 
With a Regsvr error I would suspect that it (Regsvr) is trying to write to the Registry and failing. Quite often that is a result of anti-virus software preventing the write to occur. If you have AV and it is running make sure it gets turned off when you do the install.

Regards,

It wrote to the registry OK when I installed the 32 bit version.  And originally X2 and X3a installed OK with Win 8. 
 
 

One of the fixes coming in X3c will be Windows 8.1 compatability.. maybe the update will solve the issues people are having with Sonar & Win 8.1  ; )

X3c update may fix it, but the problem, with mine, is that X3a and the X3b patch can't be installed as 64 bit version, what I want.   If X3c fixes it, I would need a complete new version with the "c" incorporated to install. Just having the "c" update won't really help me.
 
 

I have a similar problem. I had to roll back to 8.0 to get X2 and X3 to run. However the official 8.1 release will not let me start X3 / X2.  My roadblock dialog box: Missing MSVCR110.dll  ??
 
I did install X3 with virus protection turned off.
 
I filed a problem report last week. Nothing back from Cake yet.

Musicroom,  I have the same problem with the MSVCR110.dll.  It says reinstall the program but I can't because of another error.  Since I couldn't get the 64 bit version installed, I decided to use my Macrium backup, that was just after 8.1 upgrade, and do a drive restore.  That got me back to X3 being installed but I get the Missing MSVCR100.dll and to reinstall the program (Sonar).  But I can't reinstall Sonar, as it is now.   I have sent a problem report to Sonar tech support about not being able to install 64 bit version.  That was just this morning so who knows when I'll get a response.
 
I have an older Win 8 Macrium backup (before the 8.1 update).  I can use that if needed. Since Win 8 is not my main recording system I'll wait a couple of days and see if Sonar support responds with anything helpful.
 

Fireberd - I tried several times to get X3 running on 8.1. Interesting to me was that X3 ran perfectly on 8.1 preview. But I have to roll back to 8.0 via a good working backup to get the official 8.1 dl'd and installed. I thought since X3/X2 worked so well on 8.1 preview that I would not have any problems. I was wrong. 
 
So far everything I tested on the official 8.1 ran except Sonar.

I never installed the 8.1 preview, just waited for the full release. 

Did you install with administrative privileges?
That is right click installer file and select Install as Administrator. Even though you user account may have administrator privilege it's not the same. Seems a lot of computers require this of Sonar.