I did about 150km on a bike, rather intense

however, Connect shows me NO intense minutes from that day:

Most likely the file with the IM data did not sync, because it was damaged and discarded during a crash and reboot. You can connect the device to a computer with the USB cable, and check the files in the folder //GARMIN/Debug with a simple text editor like Notepad. Some of them are binary (hot human-readable), but there could be one with some verbose messages about the last reboot. On my device the file is called err_log.bak, not sure whether it is the same for other models too. If the crash & reboot happened during or after the ride, then it was very likely the culprit of the loss of the IM data. Check also other daily data (HR, steps, Calories,...) for gaps or missing data on that day.
... from Edge® 1030 | Garmin Customer Support
NOTE: The Edge Explore 2 is currently the only non-wearable Garmin device that is compatible with the Intensity Minutes feature. Any other Edge cycling unit will not be able to record intensity minutes without the HRM-Pro or HRM-Pro Plus being worn while also paired to the Garmin Connect app. Please see Can I Get Intensity Minutes From a Chest Heart Rate Monitor? below for details.
If you have a Garmin watch, just wear it while cycling - you'll get the IM's even if you do not start any activity on it. You can also keep it in the backpack, if you do not like to wear it on the wrist while riding. Just connect it to your HRM, so that it gets the same HR data as Edge does.