Upload image not working

A client of mine has a similar problem. She reports that she can apparently upload the image that then appears in the media library but then the “Returned data is not in valid JSON format” inhibits her. The ad exists and when you edit it the Select or Upload button does not respond.

On all my sites everything works as it should. In case of an error, the reason of the error must be sought individually. Perhaps you have too many images and therefore you have not enough resources to work.

Solution was a bit simpler than that. Just had to realise that it was not the latest version (even though I did not see a WP notification about this). and do the upgrade.