[Plugin: Event Calendar 3] Popup in event editor not working

Hello!

I’m using the Event Calendar 3 plugin. When I create a new post and try to add an event to it using Event editor after clicking on the “+” symbol when I click on the “…” symbol to open a pop-up to select which dates I want to set the text I wrote in the post disappears and no pop-up opens. I read the plugin FAQ that I should try to disable other plugins which may conflict with this, but when I disable all other plugins I have the same problem.
Event Calendar 3 worked ok for a few days but then it stopped. The only difference is that I enabled “set featured image” in my Arras theme.
The only modification I did to this plugin is change the name of the months (but even after I reinstall it the problem remains). I have the same problem with Event Calendar 3 for PHP 5.3.

This may be related to another problem with infobox I keep having, but probably isn’t as both don’t work when I disable the other: Thread

Can anyone please help? Thank you!