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<Error>
<Code>InvalidKey</Code>
<Message>Unknown Key</Message>
</Error>
My issue is resolved now. Thank you!
Now after your patch, I'm again able to download the single plugin versions from https://toolset.com/account/downloads/
I still haven't tried to update the plugins automatically from WP dashboard, but I think the problem had the same origin, so I consider solved also that issue. In case of problems I'll open another ticket; thanks again.