"GWCommanderClass can't get converted into GWCommanderClass"

I'm using a mix of C3PO & GW Token API. My language is C# and the Program is made for archiving into different archive storage systems.

Here my Error Message in German: 'Das Objekt des Typs "GroupWiseCommander.GWCommanderClass" kann nicht in Typ "GroupWiseCommander.GWCommanderClass" umgewandelt werden.'

Translated: 'The object of the type "GroupWiseCommander.GWCommanderClass" can't be converted into "GroupWiseCommander.GWCommanderClass".'

I got a C3PO which is using token API (GWCommander) to:
a) save the mail in mime format on the harddrive (ItemSaveMessage)
b) get the current logged in username (EnvUserID(1;0);)

My Program is first calling the ItemSaveMessage GW token from within the C3PO. Afterwards, it loads a archive dll which will do something with the eml file on the archive storage. The loaded dll is using another gwcommanderclass object to execute the EnvUserID token.
So, I got 1 commander object in my c3po dll and one commander object in a seperate dll which we replace, depending on the connected systems.

I heard, that it's recommended to free the gw commander object as soon as possible. So I set the gwcommander object to "null", directly after using it. But I still receive this weird error.

This behaviour is not 100% reproduceable.
One time it works fine with 10 tries and no error during the archiving process.
Sometimes I get this error after I archived the first item.

Thanks in advance for any kind of help/hint, regarding this issue.