Thanks for your bug report. That is because the servers are using
STS whose purpose is to prevent man-in-the-middle attacks by requiring a stricter connection policy to a specific address and port and validation of the certificate. You will need to contact the administrators to ask them to either remove the stricter requirement of STS or to fix their certificate. That said, if they remove the requirement for STS on their servers, the STS protocol requires clients to continue using the saved STS details until they expire. You would need to use the clear history option to delete your SSL and STS cache in mIRC to be able to connect to the servers with an invalid certificate once they have removed STS support.
