Recently I stumbled over the problem that the GPGOL addin of GPG4WIN (version 2.1.0) apparently has some problems with the German umlauts. At the bottom it turned out that it could not deal well with the message encoding. Here is how I managed to work around the problem.
In several posts (for example this one) and bug entries you will find users of the GPGOL plugin (“GPG for Outlook”) to complain about that decrypted emails lacking German umlauts. I stumbled over this issue as well and after this Technet Article, I found a work around which worked in my case. Apparently, GPG4WIN is not capable of working well with the ISO-8859-1 codepage (to what-o-ever reason). I my case this was the default code page which should be used by Outlookt for writing new emails. Under Extras->Options->Email format->International Settings the code page “Western (ISO)” was selected. Changing this setting to “Unicode (UTF-8)” did the job: the received emails could be decrypted including their umlauts.
Sometimes it’s just a single switch somewhere to resolve a problem that was long-nagging.