When upgrading to Trados Studio 2021 SR2 and using one of the MultiTrans Plug-Ins, you will need to make sure you are using the latest version of the MultiTrans Plug-Ins (version 3.1.2.0) which are available on the RWS AppStore
For more information on the error please refer to the following article from our Knowledge Base: https://gateway.sdl.com/apex/communityknowledge?articleName=000018343