HDMI out to second monitor stops working after logging in

Hi, I wonder if someone can help me.

All of a sudden my second monitor connected to my 2019 iMac via a USB-C to HDMI converter has stopped working. It works when I'm on the login screen, then once logging in, after about 10 seconds it just randomly goes off. Looking at the console it might be something to do with "IODisplayIsHDMISink: FALSE" happening. If I go back to the login screen, and uplug and replug the monitor back in, it comes back on. Then once I log in, it goes off again.

I usually connect a different monitor to it, a HUION tablet, and it seems to think that the other monitor is connected at first, as in display prefs, it says the name of the HUION monitor, then it realises it's not and it disappears.

I think this may have started happening when I updated to 10.15.5, but I'm not sure about that.

Detect displays does nothing, neither does resetting the NVRAM or SMC.

Any help would be much appreciated. I've never had a problem with it before.