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You have there JS conflicts:
ReferenceError: jQuery is not defined
http://www.neu.nada-akupunktur.de/components/com_k2/js/k2.js?v2.6.9&sitepath=/
Line 9
TypeError: this.filter(...).css(...) is undefined
http://www.neu.nada-akupunktur.de/components/com_sobipro/lib/js/jquery.js
Line 6
At your site are loaded multiple times jQuery libraries, with different versions. One loaded by some system plugin:
/plugins/system/sboost/js/jquery.min.js">
is very outdated version. Also you are loading some unknown version of bootstrap library. All these things can generate problems which can brake Google map displaying. You need to fix them.
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