Hello Rene,
Thanks again for your answer! A little bit more explanation now since I really need to test some things on a staging site.
So, first of all, I did what you asked and enabled WordPress debug mode in wp-config.php and installed Query Monitor.
When running WP Staging, I saw in my 9 PHP warnings and 1 PHP notice that “Wordfence” and “W3 Total Cache” were plugins mentioned. So I deactivated these plugins and tried again: 4 PHP errors and 1 notice left, but not a clue how to fix these errors. So I made this screenshot: https://static.afbeeldinguploaden.nl/1604/159743/JT3YNW54.png
Some more information which may be useful… This (https://goo.gl/tMUcoj) is my website and the exact reason why I want to use WP Staging is because I bought the Standard Package from Canvas (here: https://www.woothemes.com/products/canvas/) because apprently my theme is outdated and I want to update it (and have a year of support).
I hope all of this above is clear. If not, shoot and many thanks in advance already!
Kind regards,
Gert