What is the recommended wording for a generic error message

What would be the best wording for a generic error message?

With generic error message I mean a message for an error that has occured but there are no details on what the error is or how to recover from it.

It will be used exclusively as a fallback solution when it is not possible to determine the error either because the server did not sent any additional details or there is a "probable" timeout... and other similar edge cases.

It should be aimed to minimize the amount of frustration/anger.

I've read a few threads but none of them seems to be 100% relevant