Postmortem: we made some changes in our Cloudflare configuration to enable a higher cache hit ratio. We relied on the fact that Cloudflare explicitly states that they support URLs up to 16KB long:
https://developers.cloudflare.com/workers/platform/limits/#request-limits
Turns out it's not true: we asked for more info from Cloudflare; in the meantime, we reverted our optimization.

Our services experienced increased response times for 12 minutes between 4:25 CEST and 4:37 CEST today. The issue was promptly identified and resolved by our engineering team. Please note that the status page graphs incorrectly show a +50-minute degradation period due to a misconfiguration in our reporting system. This does not reflect the actual duration of the incident. The reporting system has been corrected. All services are now operating normally with regular response times.