I'm also having similar problems, but what I've done to overcome this issue is click on Media and nothing allowing selection or additions, I'm using the add media by URL additions or basically click on Media to add and select a new image
Then I Right Click on Image and Copy URL that I've uploaded using Joomla built-in Media Selector or plugin in my case
and finally back out of file requester and from the main Page Builder interface I Paste the URL copied earlier and Add Image to default selection Page Builder provides.
Yes this isn't quick, but it's a work around till Saguaros gets back with us with a fix. Sorli...