Thanks everyone for reaching out and putting together a detailed report! 
This error is due to high workload at the server. We currently serve up to 64 requests concurrently; that number is exceeded during ‘rush hour’. Typical times are at the top of the hour, at xx:15hrs, xx:30hrs, xx:45hrs; at the beginning of each minute; etc.
Everyone, be a good user and schedule your cron jobs at randomly picked moments of the hour and minute, avoiding the rush hour, don’t send no-op requests.
We will discuss on GitHub if we can increase the number of requests we can process in parallel, but this directly translates into memory usage, which directly translates into server cost Euros. 
Perhaps we need to further limit the amount of requests a user can make so that people stop sending us no-op requests that just result in busy work (read: unnecessary cost) at the server.
Best solution would be if someone wrote an update client that deserves the name and reduces the number of no-op requests and randomizes query times.
Best,
Nils