This is log message has severity NOTICE which means it is a non-error, however it seems either this is a real error with the wrong severity, or it should be disabled in production.
+145,000 hits in the last 24 hours, during enablement of 1.32.0-wmf.18. In the diff channel.
level=NOTICE type=mediawiki channel=diff Falling back to DifferenceEngineSlotDiffRenderer
Example url that triggers the message in the logs server-side, https://en.m.wikipedia.org/wiki/Special:MobileDiff/243891470.