Yep, another one. If you use custom tasks in SSIS, the dll containing them needs to be deployed in the GAC in order for SSIS to be able to find and use it. When I tried to add my custom task, I got the following error

Failure adding assembly to the cache: Unknown Error

which, again, proved to be invaluable information in the debugging process 🙂 Luckily, this one turned out to be real simple: the development machine I tried this on, didn't have the .NET 2.0 SDK installed, so I had just tried the gacutil of the 1.1 version. Installing .NET 2.0 and using that gacutil version did the trick