After some searching I found a Microsoft KB article 182500 stating that fm20.dll might be missing or incorrectly registered. This KB applied to Excel 97, but apparently it was still happening in Excel 2000. In my case fm20.dll only needed to be registered with the command regsvr32.exe fm20.dll, which I started from the run window.
I pressed OK and a notification came up saying the dll was succesfully registered. When I started Excel the error was dissapeared.