I've not tested this extensively but I was having some issues with Krita last night, and it all seemed to be triggered by having two files open at once.

First, my OS and driver info:

OS: Windows 7
Graphics card: ATI Radeon HD 5450
Driver version: 8.841.0.0 (updated to the latest a few days ago)
Driver model: WDDM 1.1
I use a MS optical mouse, not tablet and pen

I launched Krita and set up a new canvas, then I opened a jpeg photograph to use as reference. Almost immediately Krita started behaving oddly - I couldn't paint and many of the settings like brush size and opacity were greyed out. I wasn't on an eraser or blend tool, I had a strong colour selected, and I made sure I wasn't working on a blind layer by mistake. Shortly after this Krita crashed and told me it needed to close.

I relaunched and tried again with a different photo, and although things were fine initially, the same thing happened after some time. Other odd behaviour was related to a couple of images I used to create custom brush-tips - in that they were being applied to the canvas as a solid colour (whatever colour I had selected) rather than the image itself.

I did eventually manage to complete my piece, with two files open at the same time, but for whatever reason Krita certainly didn't like it the first few times I tried.

In actual fact I made a mistake in saying I eventually got Krita to work with two files open at once. This is not the case when I think back. In the end I had to have my canvas open in Krita, and my reference photo open in my default image viewer, InfranView.

I've tested this further now and Krita will not allow me to have two files open at the same time. It starts misbehaving almost immediately and inevitably crashes shortly after.

That's pretty weird... On my win7 system I can have dozens of images open, but that's with an nvidia card. Maybe that makes a difference. If you can make a backtrace following https://docs.krita.org/Dr._Mingw_debugger then we might be able to figure out where the crash happens -- in Krita, or in the graphics driver. Did you also try to disable opengl to see whether that makes a difference?

Thanks for the advice.

I'll try with opengl disabled (where will I find that please?) and get back with my results.

It's not a massive problem as having reference material open in my default image viewer suits my needs just as well.

settings/configure krita/display

Okay thanks.