Unable to open audio playback device - X3 Producer

This should be simple but I can't see it!
I have a project that I'm halfway through.  I bought my first ever microphone and got it to work with my UA-25EX on a separate project.  I've come back to my first project and now it fails on "Unable to open audio playback device" - I get this as soon as I hit play and in addition, it won't audition loops in the browser.  I tried opening a new project and was able to browse/audition loops.  I also tried all of the different sample rates available via the switch on the back of the UA-25EX, matching them with corresponding changes in Preferences/Driver Settings.  No luck.  I've also tried a reboot.  I feel like a proper idiot - this should be simple!  Please help!
edit:  I've got another project works fine, even when I load it up alongside the problem project.  Full error message is:-
 
"Unable to open audio playback device. Device may not support the current project's audio format or may be in use.  For help, visit: www.cakewalk.com/support/hardwaresetup/ or contact Cakewalk Technical Support"
 
 

Zargg71
Hi. It this an USB mic? If so, maybe that is the problem. If you use ASIO driver, you cannot use 2 (both mic and the UA-25EX) instances at the same time. If that is not the case. Do you have share drivers checked in Preferences (P), Audio, Playback and Recording? If so, try unchecking it. Maybe Windows or something else has taken control over your UA-25EX. That is all I had for now. Hope it helps.
All the best.

Hi Zargg
Thanks so much for your suggestions.  No it was not a USB mic, it was going into the UA-25EX; I had to fiddle about with the interface a fair bit to get it working which is why I thought I'd messed up the settings.  But the thing is, a new project works no problem and older projects work no problem, it's just the project I was working on just before the new mic arrived.  I also changed the "share driver" settings in Preferences as you suggested but no joy.  I thought you might be onto something with that because at one point I definitely saw the audio interface referred to as "UA-25EX (2)", however that went away, now it's "UA-25EX" again and I still have the same problem.
 
I'm thinking I must have somehow got a corrupt project.  The only thing I can think of now is to move everything over to a new project track by track.
 
I appreciate you taking the time Zargg - Tusen Takk!  Is there anything else you can think of?
 
Anyone else seen this before?
 
 

Hi Ken,
I tried opening in safe mode with all the plugins disabled but I still had the same problem (but thanks for that, I didn't know about that trick).
I'm now in the process of recreating the project.  I've got the old and new side by side; it's time consuming and annoying but has to be done.
Really appreciate your help.
Tusen Takk!

If you have many clips, maybe try to bounce each track to clip(s) in the old project before dragging / copying them over to the new project?