Error Log ob_end_flush

As it occurs during shutdown, it’s of no consequence, but I get that it’s annoying. It’s probably due to a theme or plugin doing the same thing, so there is nothing left to send. Narrow down the cause by selectively deactivating plugins or swapping themes until the notice is no longer logged. Once the module is identified, take up the issue through that module’s dedicated support channel.

There are some other possible solutions on stackoverflow. I cannot attest to the appropriateness of any solutions presented there.