I was meanwhile able so solve this issue, and it might be helpful for somone else: Sorting and adding galleries to an envira album kept producing “undefined” error 403 forbidden messages from wordpress admin-ajax.php. The error message was due to the (not unusual) php server config and could be solved by increasing php values max_input_vars, max_input_time, max_input_nesting_level via htaccess file. good luck.
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This reply was modified 4 years, 6 months ago by julmoel.
