Kitty - Assertion Failure

I am plagued by getting 'assertion failures' (image posted at https://itproxy.uk/misc/assertfail.jpg) when trying to use Kitty. It can happen constantly, or occasionally, when I'm trying to visit websites using Firefox Portable (currently v77.0.1) on a W10 PC, via Kitty Portable (v0.74.0.2) through freeSSHd (v1.3.1) installed on a W10 PC.
I'va absolutely no idea what the error message I get means. Where's the issue? With my browser? My client ie Kitty? The SSH server? The website I'm trying to visit? Or the PCs involved?
Any help with where/how to start investigating this issue would be appreciated.