you have to delete all hunkusage.dat files from your call of duty 2 first. first go to "hx folder" and find hunkusage.dat and delete it similarly delete all hunusage.dat files from other folders too. i.e from "h4x-zombie" and "xX-h4x-Xx" folders and main folder too. look in every other folder of your call of duty 2 if you find any hunkusage.dat just delete it. only connect to server if you have deleted all hunkusage.dat from all folders of your call of duty2. if you accidentally left any hunkusage.dat before connecting to server then this error will came back again. hunkusage.dat will download automatically when you connect to server. if you got same error again then you may have left any hunkusage.dat in your call of duty2. you have to delete all hunkusage.dat again.
2nd solution is to reinstall the whole game but before reinstall you have to delete all folders of call of duty. there should be no file and folder left in call of duty 2. if you left the error will came back again. so before reinstalling properly delete call of duty 2 whole folder.