On October 15, 2002, Codemasters release a patch to fix problems when playing the Red Hammer campaign (specifically mission 6, "Under Fire") for users who have installed OFP: Resistance and the 1.85 upgrade patch. A little more than a month later, an equivalent patch was released for OFP version 1.90. On March 28, 2003 a new version of the Red Hammer compatibility patch was released for version 1.91.
Users who installed OFP from the OFP Resistance CD must install the Red Hammer compatibility patch appropriate to their installed OFP version. For example, if you upgraded from OFP version 1.75 to 1.91, you can only install the 1.91 edition of the Red Hammer compatibility patch.
Users who installed OFP from the OFP GOTY edition CD (see this FAQ item), should not install any of the released Red Hammer compatibility patches. The corrections included in the Red Hammer compatibility patch were incorporated into the OFP GOTY edition at production time.
Users who upgraded to OFP version 1.90 or 1.91 and nevertheless tried to install the newest Red Hammer compatibility patch when they had previously installed an earlier version of the patch, found that they could not install the newer Red Hammer compatibility patch. Worse, the patch's execution error resulted in the corruption of the Red Hammer's campaign's PBO file.
To resolve this problem, simply restore the original copy of the REDHAMMER.PBO file you hopefully made a backup copy of. You will then be able to install the Red Hammer compatibility patch without a problem. Didn't make a backup copy of file REDHAMMER.PBO? Tsk, tsk! You better contact Codemasters support and see what they can do about it.
You can download the Red Hammer compatibility patch from BIS' download page.