I'm asking on behalf of employees who use the drone, so I don't have the exact errors etc. but here's the gist of it:

They try to fly up along a 300m tall mountain, but get warning after 150 meters or so, and at 250m(?) it refuses to fly higher.

How are these restrictions working? What is the max altitude, and does the restrictions apply to sea level or ground level?

This is in a rural area without any flight restrictions. We're just inspecting the mountain, to find a landing spot for a helicopter at the top of the mountain.

We don't need to fly more than 100 meter above ground.

Having screen shots of the warning would give us a better chance to answer the question

As for height restriction they apply from the take off point as that is zero.

The Mavic has a max height restriction of 500m but is usually set to 150M as 400ft is the max height in most countries, the app has a section for max height settings, the warnings may also be caused by him losing signal if the quad is close to being overhead.

Ok, so if you adjust the setting to 500m, it will allow flights 500m from take off point (unless in a restricted area)?

wmo12 Posted at 4-20 05:18
Ok, so if you adjust the setting to 500m, it will allow flights 500m from take off point (unless in a restricted area)?

Yes
The drone has a hardwired altitude limit of 500 metres above the launch point and a user-configurable Max Altitude limit which is by default set at 122 metres (400 ft) above launch point.

The Max Altitude limit can be set to any height up to 500 metres.

Hi wmo12, the max flight altitude is 120m by default, but it can be set in DJI Go 4 APP up to 1640 feet (500m) except in flight limit zones, this height is relative takeoff height. Please always follow the local rules and regulations.