Hi Philipp,
It sounds like you're experiencing some synchronization issues between your HX Stomp and Ableton Live when using external tempo sync. This can be frustrating, especially when you're trying to maintain a consistent tempo for your endless delay.
One potential solution is to ensure that Ableton Live is sending MIDI Clock to the HX Stomp. You can do this by going to the Transport menu in Ableton Live, navigating to Project Synchronization, and selecting MIDI Clock as the destination. In HX Edit, make sure the tempo sync is set to "external" so that it follows Ableton's tempo.
If this doesn't resolve the issue, you might want to check your MIDI settings in both Ableton Live and HX Stomp to ensure they are correctly configured for synchronization.
I hope this helps!
MyFedLoan