When you use Mapper from the BizTalk Toolbox, and you click on Edit next to BizTalkMapClass, you may receive an error message as shown below. This error doesn't occur in previous BizTalk versions.
BizTalk Mapper Activity
Failed to locate the project that contains the document "".
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
This problem is fixed in the following cumulative update for BizTalk Server:
Cumulative Update 1 for BizTalk Server 2020
Note: To fix this issue, Developer also needs to install the latest version (>=188.8.131.52) of BizTalk Server extension from Visual Studio Marketplace.
For information about the service packs and cumulative update list for BizTalk Server, see the following article in the Microsoft Knowledge Base:
2555976 Service pack and cumulative update list for BizTalk Server
For more information about BizTalk Server hotfixes, see the following article in the Microsoft Knowledge Base:
2003097 Information about BizTalk hotfixes and Cumulative Update support