Same here. I got the same error message.

Could anyone help?

We have the same problem and also need an urgent fix, please help! Already installed the compatibility package again, but the issue still exists.

Hello Stéphanie Paradis ,Ricky Lee Oliver Bettzieche

Please note we have released a new version for SDL Trados Studio 20201

Simply remove the old version and install the version specifically for Studio 2021 SR1 version number 16.2.1739

Thank you, Steven.

It works now.

Great, it worked!

However, I still have to open the WS projects manually in Studio. Usually, when I downloaded them and clicked on them, they would open automatically in Studio, but since I upgraded to 2021 it doesn't work. Is this something that can be fixed or I will have to always open them manually from now on?
Thanks a lot for your help.

Hello Steven
how can I download version 16.2.1739?
In Studio, the "Check for updates" option doesn't show any new version.
On the SDL website, the available version is SDLTradosStudio2021SR1_16.1.2.4090.exe
Thank you.

Riccardo

Hi Steven,
I have got the same problem: just as Riccardo the latest version on the SDL website is 16.1.2.4090, no new version shown in my Studio. Installing 16.1.2.4090 did not fix this issue.

Stéphanie Paradis, 
Thank you for this question.
And thank  for the solution.

Roland Tubée
Try this one: 16.2.1739 
It is the WorldServer Compatibility plugin to be updated, not Studio itself.
Also, it is not obvious how to 'uninstall' the older version. For this purpose, you should uncheck the WorldServer Compatibility item box in the list of installed plugins. Then just run the new plugin once you have it downloaded.

Steven means you should update the plugin, not Studio. This one: appstore.sdl.com/.../