App Engine Flexible: Timed out waiting for the app infrastructure to become healthy

According to the gcp team, this particular error was caused because we reached the "In-use IP addresses" quota limit.

They also said that are working on improve the error messages.

"The engineering team has just created a fix for better quota error details. There is no ETA for when the fix will be released, but I would guess it would be in the next version of gcloud."

Over half a year later after Loneck's answer I've got the same error. I guessed it could be anything. This is why I've choosen to delete the project, create a new one in a new zone and deployed it there. Then it worked for me. It might have been any other limitation in the zone that I've choosen at the beginning.