Hi Maurits,
Disabling this wont help fix the error but it will hide it away. I'm not a Wiki expert, but if you go into:
/mod/wiki/parser/markups/wikimarkup.php and search for protected function generate_header, then a few lines below that you have:
if ($level < 4) {
If you change this to:
if ($level < 1) {
...the Table of Contents will disappear from ALL wikis. It should come back if you reverse the changes.
Regards, John