I'm not sure what's causing this, so can't list any steps to fix it. Normally DCC problems affect the ability to send files, so 1 thing to do would be to make sure the problem isn't really that the other person can't send.
One thing that can affect the ability to receive files is PASSIVE DCC. In order for someone to send, they must use ports forwarded in the router. Normally, DCC SEND uses ports forwarded by the sender, but with PASSIVE DCC it uses the receiver's forwarded ports instead. So, if your own ports are no longer forwarded, then the other person's sends can't use your forwarded ports. You didn't say whether or not you can send too.
You didn't list the symptoms. Is there just no response, or do you see the incoming file that just stalls-out before eventually failing.
You might try coming to #mirc on libera.chat or efnet and see if someone can see if they can figure something out.
Another thing that could cause the problem is that a script can interfere with this, so try temporarily disabling events with
/REMOTE OFF
then see if it works. But then be sure to turn back on with: /REMOTE ON
Another thing that can cause the problem is if you have them in your ignore list so that you don't see either the NOTICE or CTCP message associated with a DCC SEND, so see if they're showing up:
/ignore -l
