I asked the guys at CB and they said that an extension must add the following variable to the links of scripts. Is that the template doing it?

You can see that in Google dev tools at the network monitor when you open the registration form at kite-and-windsurfing-guide.com

This is not something the template does.

It is likely related to a system plugin so I would suggest turning all your 3rd party ones off to see if that fixes it, and then turn them on one by one.

I did that and when I finally switched to protostar instead of buildr the &_=1491531458355 is not added. So it is the template. Please help finding the buildr setting that causes that. I already disabled all optimisations.

Switching to protostar removes all the modules that are assigned to positions in the buildr template. So switching to protostar does not automatically mean that the template is the issue.

The template does not, and cannot alter the urls called by other components.

I would suggest looking at the extension you are using to create the model popup.

Since obviously the customers have to do the testing ... as I pointed out it is your code that adds that variable onto links. I was hunting your bug all day and just now I was able to pin point it to the "Replace jQuery" setting. When this is set to YES the variable gets added.

Our code does not add any query strings to any links.

If you chose to replace jquery with something other than the joomla version then you are responsible for the url that you add into that input.

It sounds like jquery was causing your problem.

Thanks for posting back your solution

With testing - we can't test every setting with all the many different components available

Best of luck with the site

Regards,
Paul

You are also adding another version of jquery to your pages outside of the template or joomla. This may be your issue.