Failure adding assembly to the cache: Unknown Error – GACUtil
gives you: Failure adding assembly to the cache: Unknown Error
If you come across this error when attempting to add an assembly (such as a Sharepoint Web Part) to your Global Assembly Cache (GAC), one possibility could be that you are not using the GACUtil utility for the correct .Net Framework version you need (check the path to your GACUtil utility).
If the above is true, install the .Net Framework Software Development Kit (SDK) for the .Net Framework version you require, then use the GACUtil that comes with this.
(I was attempting to use GACUtil from my .Net Framework 1.14.. directory. There was no other GACUtil on my system. My default .Net Framework was v2.0 – downloaded Framework 2.0 sdk and bang, used the GACUtil that came with this successfully).