Seems it was a problem with asynchronous loading of jquery.js and colorbox-JS.
Loading of jquery.js was sometimes completed after the loading and execution of jquery-colorbox-wrapper.min.js.
Due the reason that jquery-colorbox-wrapper.min.js was loaded by me with the async-attribute in the <script src…”-tag (manually coded in function.php) the error happened.
So now I have added a “defer” attribut to jquery-colorbox-wrapper.min.js and it works fine.
