Adding a new Script crashes Photon Namespace (VS Code)

As the title said, if I add a new script to the game, the Namespace from using Photon.Pun; got broken, all scripts are flooded with errors - but all compiles without issues. Even the game is runing.

If I load a backup, all is runing fine, until I add another script :frowning:

What can I do to avoid this issue?

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I am using
Unity 2020.1.2f1 Personal
Visual Studio Code 1.46.1 (with Addon Unity Codesnippets and Unity Debugger)

What’s the error?

All errors refer to the Photon scriptparts like RPC, Instantiate, … But the script is still compiling and the game is runing. But with all those “false-errors” it is very hard to track down real errors.