No, the beta does not fix it. Is there any debugging flag that I can enable to help you on this?
Thanks for testing it out.
Until I know what the cause is, there isn't really anything you can do on your side.
I am going to create several betas, at different stages of development from 7.55 onwards, to see if we can narrow down at which point it starts to produce this error. I will let you know when I upload the betas.
Question: on the off chance that this may be related, are you using anti-virus software? Anti-virus software can occasionally treat new versions of mIRC differently until the anti-virus developer updates their detection files. If you disable your anti-virus software completely and test mIRC, does it show the same error?