And if my script says /scid $cid, and the identifier $cid is provided by mIRC, and the cid mIRC gives me for $cid doesn't exist ... Sounds like an mIRC issue to me.
As I keep saying the use of /scid $cid most likely isn't the cause of the bug itself, it just shows a misunderstanding by the scripter in how it works and suggests there may be other issues arising from other incorrect uses of it.
There are hundreds of thousands of people using mIRC and thousands more using scripts which make use of $cid and /scid, the only person having this issue is you. Clearly that puts the likelihood of the problem being mIRC very low on the list and points squarely at an issue in your script. I've already told you how to verify the offending line (place echoes before each use of /scid to verify the values being passed to it) and you don't seem to have done that yet. I don't know what you expect us to do if you're not willing to help yourself and us to determine the true cause.