I found out what caused the problem: in my script I used the class Mappress_Options for temporary overwriting the settings from the options page – but Mappress_Options->save() also deletes the API key, if not set in the options array! Commented out these two lines – and the API key stays in the database:
$myOptions = new Mappress_Options(array(“initialOpenInfo” => false, “tooltips” => false, “autoCenter” => true));
$myOptions->save();
For my project – solved. Nonetheless an unexpected behaviour.