U might be accidentally pressing a function key. Next time it happens try hit, I think it's f2 ? Probly wrong just keep hitting function keys till something works or breaks

Was it Tab?

I had the exact same conundrum. Press F2 to close it.

To take a screenshot press F12. My game is somewhat bugged when it comes to taking screenshots. In the start menu screen when I log on to Valheim, if my Shift-Tab does not bring up the Steam overlay for chat, friends &c, then I will not be able to take screenshots during my gameplay. To fix this I simply have to log off then on again and the Shift-Tab will work and I will be able to take screenshots.

After taking a screenshot, to post it, in the upper left corner of your Steam overlay, just above the STORE option, you will see "View". click on View and the fourth option from the bottom will be "Screenshots". Open that tab add your screenshot to your online library, then chose to share. Along with options to share to Facebook, Twitter, &c, will be an URL web address, copy then paste that into your forum post to display your selected screenshot.

Hope that helps, but I am sure hitting F2 will fix your dilemna.

"Quando Omni Flunkus Moritati"