That appears to be a numeric sent by conferenceroom, so it would appear the DCC SEND message the server receives from your client has become.. 'whacky'. Do you have issues sending files on any IRC network (you wont receive the same error, but the file wont send)?
Try '/debug -p @debug', then attempt to resend the file and look for the PRIVMSG line you are sending ('->') the server. If those characters appear in the line then something is very wrong with mIRC. However I suspect the problem lies with a router or similar incorrectly 'fixing' your DCC sends to use the correct IP while behind NAT (you will not see this yourself, as it is changed after mIRC sends the line). If this proves to be the case you will need to try upgrading your router/modem/whatevers firmware or seeing if the vendor can provide a fix.
Hope this helps - I could be totally off track though