Resolved. Our infrastructure provider has closed their incident and all regions are reported fully operational.
Summary: tasks that run code, generate files, or execute shell commands failed to start between 01:44 and 02:01 UTC (approximately 17 minutes), with reduced reliability and slower start times continuing until 02:16 UTC. Since then we have recorded no failed sandbox provisioning attempts and no elevated latency across several hours of normal traffic.
Everything is working normally....