Well it seems to be an error reply from the server, on UnderNet it was raw 417.
<- :<server> 417 RoCk- :Input line was too long
I made that error by typing on a channel //say $str(t,492)
492 characters caused the error, 491 characters did not. It may be even less on your network. When someone requests a file from you, OmenServe sends a confirmation notice, in that notice is the file name, the queue position and other information and a lot of color codes. All of that information plus a long file name could be exceeding the servers message length limit. Also it could be your skin, if you have added a long message before your main advert, and maybe before and after random play messages in the skin section (F2 > Skins) of OmenServe options, this could be causing the error too. Either way, it's not a bug in mIRC or OmenServe.
~ Edit ~
I don't think most servers even send an error reply, they just truncate the message. I've never even seen this on UnderNet before, it must be new or I've just never exceeded the limit.