Meraki put an modified firmware on our MR45:
"So, I need to move just this MR45 onto a different firmware for now, this will mean this will allow everything to keep working even if your MR is rebooted. Again this is only temporary whilst the entire issue is looked into on our side.
Please note the firmware change will involve a short amount of downtime as the device reboots 2-3minutes is likely.
If this is also agreeable I will make this change from my end?"
It´s working now.
To remember:
The issue occures (only?) on Wireless-AC 8260 and 8265 wireless hardware
The issue occures only when the notebook or tablet is running on battery, no issue with an external power supply
The issue occures only when the power saving of the WiFi hardware is set to standard (Win10), not when you change it to high power consumption
In the End: you can´t put blame on Meraki. I think it´s an Intel issue.