VM + SEF + cache + pagination problem

[sandomatyas]

I manage a shop (Joomla 3.6.0, VM 3.0.16) where I enabled SEF links and cache.
A category url looks like: mydomain.com/category1
Everything works well except the pagination. I display 100 products per page and the url of the 'next' button is mydomain.com/?start=100
I don't know where it comes from but it redirects me back to the main page.
I checked the VmPagination helper where the _buildDataObject() function builds the url: $data->next->link = JRoute::_($params . '&' . $this->prefix . 'limitstart=' . $next);
The link without using JRoute is: &limitstart=100
After JRoute is: /?start=100

I also checked $this->_additionalUrlParams array which is empty.
I tried to check URL variables like option, view, virtuemart_category_id and if I push these to $this->_additionalUrlParams the link becomes: mydomain.com/category1/results,101-100

Have I missed something?