Hello, I opened a case for this problem.
It´s only on our MR45 (not on MR42, MR42E, MR52, MR32 ...)
AND
It´s only on some Dell Notebook models and on Microsoft Tablets (not with Apple Notebooks f.e.)
The problem occures only when the devices are running on battery.
If you plug in the power supply the problem disappears immediately.
If you change the Win10 power-saving for the Wireless adapter from "normal" to "high..." then it works!
Dear Meraki stuff, do a update for this accespoints, and tell the service that this problem exists, they don´t believe me!