Can you please try disabling the viewstate MAC validation for pages and see if you still reproduce the issue?
To this please open the web.config and locate the
Add the following attribute to it:
Note: you might have 2 <pages> nodes in your config file - make sure you add it to the one that has <controls> as a child item.
Then you need to redeploy the project to Azure.
Can you please provide more information about the distributed session provider and how you set it up?
Thanks a lot.
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