Jim, you may be glad to hear this. - Or you may be infuriated because it is what you're always going on about, - ie, trying to duplicate a problem on one of
your machines, that is really a fault on a customer's machine!
Bottom line is... Things are now working as they should!
I was running out of things to check, and of programs that would scan my system for potential nasties. - I had spent the best part of two days trying to clean out every nook and cranny. - So I decided to roll-back to an even earlier 'Image' I made on the 20th of last month, - 11 days before this problem started.
At that time DA was at v1.181, and MA3 at Beta7c. and everything worked as expected. - I then updated to the current DA 1.186, and MA3 Beta7h, and everything still works as it should. - So I will probably never know why MA3 killed DA, and why MA2.6 brought it back to life!
Calle, - just for the record, (though it doesn't really matter now), - The Official versions of DA do not have the one-scene-across-two-monitors option. They are two separate 'aquaria'.
(Posh word that! 
) - And it was one image across two screens that was affected, not a 'single-screen' presentation.
Though I may never get to the bottom of that DirectX Error Message, - I felt sure that my system must be involved in some way. Why else would it suddenly start going wrong on that particular day? - But I'm convinced also that there's some connection between DA's recent betas, and MA3Beta7's, both having backgrounds across two screens! ... And why did MA2.6 correct the problem?
And why did a roll-back to an image made a few days earlier not solve the problem that only started that day! - But an even earlier image did?
DirectX is tied up somewhere in all this, I'm sure of that!
Ah well.... Water under the bridge I suppose!
As regards Beta7h.... I see no difference with the monitor refresh, but that wasn't really a problem for me. - What was more disappointing was I was hoping it might solve the DirectX Error Message problem. - But that, thankfully, is now moot!