I cannot reproduce this error on my end.
And per my research, I only find this issue related with this error: https://stackoverflow.com/questions/68894869/sharepoint-unfamiliar-error-on-kw-search-substratesearchexception-remote-execu There someone said, they made it work by adding a filter in the query, maybe you could have a try.
For the reason why this error happens, I would suggest you open a ticket with Microsoft to check more.
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