[Map][Tooltip] Issue displaying biggest customer DAX/formula - map point limits

Hello PowerBI community,

I created the following Measure

Biggest Customer Name = Lookupvalue('Atlas kurz'[Customer - Global HQ]; 'Atlas kurz'[OR Total]; 'Calculated Measures'[OR Total max per Customer])

to return the name of the biggest customer by orders received in the selected region.

This works totally fine for cards, but wheb i want to add it to tooltips, so it shows the biggest customer of a city on the city's tooltip, the map object crashes with the following error message:

The previously used measure looks like this

OR Total max per Customer = 
MAXX(
	KEEPFILTERS(VALUES('Atlas kurz'[Customer]));
	CALCULATE(SUM('Atlas kurz'[OR Total]))
)

A seperate problem to this occurs when using cards: 

The City goes blank when the Customer with the determined OR Total is not mapped (at least it appears so, i tested it once with whole country (name goes (blank) and then with region of the customer (name was shown)

Is there an elegant to work around this? 

Any help is highly appreciated. :)