@rossko57 - Thanks for pushing me in this direction.
In openhab.log DEBUG of icalendar binding shows me every 6 hours a timeout:
2022-03-07 19:22:50.481 [WARN ] [g.icalendar.internal.handler.PullJob] - Download of calendar timed out (waited too long for headers).
2022-03-07 19:22:50.486 [DEBUG] [g.icalendar.internal.handler.PullJob] - TimeoutException message is: null
2022-03-08 01:23:50.491 [WARN ] [g.icalendar.internal.handler.PullJob] - Download of calendar timed out (waited too long for headers).
2022-03-08 01:23:50.495 [DEBUG] [g.icalendar.internal.handler.PullJob] - TimeoutException message is: null
2022-03-08 07:24:50.500 [WARN ] [g.icalendar.internal.handler.PullJob] - Download of calendar timed out (waited too long for headers).
2022-03-08 07:24:50.502 [DEBUG] [g.icalendar.internal.handler.PullJob] - TimeoutException message is: null
2022-03-08 13:25:50.506 [WARN ] [g.icalendar.internal.handler.PullJob] - Download of calendar timed out (waited too long for headers).
2022-03-08 13:25:50.513 [DEBUG] [g.icalendar.internal.handler.PullJob] - TimeoutException message is: null
Every 6 hours (=360 minutes) is exactly the interval I defined.
Downloading of the URL within browser and cURL works as expected.
How can I deeper analyse why the icalendar binding run in timeout?