"BPS_readonly" does not exist, this type of principal cannot be impersonated, or you do not have permission

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In the browser, the portal WEBCON shows the following error:
Cannot execute as the server principal because the principal "BPS_readonly" does not exist, this type of principal cannot be impersonated, or you do not have permission.

the BPS_readonly account exists in DB\Security\Logins and has access to the Content database.

How to fix it?