This now led me down the problem of there being no debug log: apparently that is a known error in windows (https://community.icinga.com/t/windows-agent-enable-debug-log/5735)
[2021-10-13 15:45:27 +0200] information/FileLogger: 'debug-file' started.
[2021-10-13 15:45:27 +0200] information/FileLogger: 'main-log' started.
[2021-10-13 15:45:27 +0200] information/ApiListener: 'api' started.
[2021-10-13 15:45:27 +0200] information/ApiListener: Started new listener on '[::]:5665'
[2021-10-13 15:45:27 +0200] debug/ApiListener: Not connecting to Endpoint 'xxx' because that's us.
[2021-10-13 15:45:27 +0200] debug/ApiListener: Not connecting to Endpoint 'xx' because the host/port attributes are missing.
[2021-10-13 15:45:27 +0200] notice/ApiListener: Current zone master: xxx
[2021-10-13 15:45:27 +0200] notice/ApiListener: Connected endpoints:
[2021-10-13 15:45:27 +0200] information/ConfigItem: Activated all objects.
[2021-10-13 15:45:27 +0200] notice/ApiListener: Updating object authority for objects at endpoint 'xxx'.
[2021-10-13 15:45:27 +0200] debug/IcingaApplication: In IcingaApplication::Main()
[2021-10-13 15:45:37 +0200] information/WorkQueue: #4 (ApiListener, RelayQueue) items: 0, rate: 0/s (0/min 0/5min 0/15min);
[2021-10-13 15:45:37 +0200] information/WorkQueue: #5 (ApiListener, SyncQueue) items: 0, rate: 0/s (0/min 0/5min 0/15min);
[2021-10-13 15:45:37 +0200] debug/ApiListener: Not connecting to Endpoint 'xxx' because that's us.
[2021-10-13 15:45:37 +0200] notice/ApiListener: Updating object authority for objects at endpoint 'xxx'.
[2021-10-13 15:45:37 +0200] debug/ApiListener: Not connecting to Endpoint 'xx' because the host/port attributes are missing.
[2021-10-13 15:45:37 +0200] notice/ApiListener: Current zone master: xxx
[2021-10-13 15:45:37 +0200] notice/ApiListener: Connected endpoints:
[2021-10-13 15:45:47 +0200] notice/ApiListener: Updating object authority for objects at endpoint 'xxx'.