Oke now I'm totally confused.
So I thought if it could be templates, lets install it on my test forum where I've got totally no addons, not changed anything in templates and it works perfectly.
So I thought, lets install it on the live forum again and see what is causing the issue. And now I can't reproduce the error on my live forum either anymore? Only thing I did was uninstall this addon this last night and added it again just now.
No changes made to templates, styles or addons in between. And now it works without any issue.
Odd... so I'm sorry, can't reproduce it now, but if it might happen again, I will post then.