It worked very good and pretty, with different parts of the site assigned to different template colours etc., but because the template version 3.0.0 did not work with PHP 7.1, we installed/updated to 3.1.0 (framework: 3.2.3). After doing that the whole front end is giving a http error 500: it doesn't not show anything now.
We have tried to install the plg_jsntplframework_3.2.3 once more, switched between php 7.0 and 7.1 and disabled/enabled some plugins and extensions, but it makes no difference.
When we switch the template to f.ex. the built-in Joomla Protostar, the site shows up again (without the modules and menu that do not have the same position in that template), so that might indicate the template does something unexpected.
Our webshop needs to be online again as soon as possible... We don't know where to look anymore: what can be the problem and how can we fix it?