if i am indoor - no gps signal - the dji says ..max hight allowed 10 m, but if i go over about 3,5 m it says limit reached and does not go higher.
What can be the reason ?

Hi, thank you for reaching out. We do not recommend flying indoors using your DJI Mavic Mini as this might cause some issues and instability since the unit cannot detect a GPS signal. In addition, kindly please try to uninstall and reinstall the DJI Fly app on your mobile device to see if the issue persists. Thank you for your valued support.

Hi Gunter,

It is written in the manual, page 37. Height limitations is 5 or 30 meter.

Flying indoor with good light conditions and good reference of the surface below the drone, no problem.
Mini uses its vision system to fly stable.

Re-installing the FlyApp is useless for this, as your drone acts like in the manual.

cheers
JJB