Hello Maarten
Those RewriteCond entries are cumulative and it looks like there are missing commands.
Who edited that file last, and what did they change?
The way conditionals work is you set a condition, Then you add a command, then a line break You can add more than one condition and it acts as AND
This page has what look like too many conditions and not enough commands -but it could be ok
Try adding a blank line between the rule entries and the Cond entries (but not between the Cond and the Rule entries)
Here is what to do to test anything like this: Save a copy of the .htaccess Then start editing it Delete everything below ##Start rewrites See if that fixes it. If not, the problem is above or if that fixes it, the problem is below Keep cutting the file in half or adding half until you discover the problem line
It is harder with all those conditionals, I suspect it is the lower block that is the problem
So remove those Cond entries from the bottom up