I recently find that Trados 2021 or some plugin (or they togeter) will damage the sdltm file that is checked for update in the Project Settings. The problem is that sometimes a message shows that "searching in TM error" (general meaning) during translation. Most of time the sdltm file checked for update works well in the editor, it can be updated and searched normally. However, when I try to seperately open this sdltm file using Trados "File" - "Open" - "Open TM" menu, an error message appears saying that "error occurrs when searching in the TM which may contain invalid translation unts." as shown below. 

Hope I could get help from you friends, thank you very much.