What did you do to cause this?
Create a main wiki page with a relative link to another wiki page, e.g. page2
Now create page2
If you now open the main wiki page by clicking Wiki in the main menu, the link to page2 is projectname/page2, which is incorrect.
What page were you on?
projectname/wiki (bad) and projectname/wiki/main (good)
What PHP and MariaDB versions do you run?
PHP5.4 and MySQL (sorry...) 5.5 ;-)
Describe the defect:
If you just click Wiki, then the path-segment 'wiki' is missing in the relative links to other wiki pages. (projectname/page2)
If you click >Wiki >Pages >Main
then the links are correct: projectname/wiki/page2
The problem is just that the same page is displayed with 2 different urls, without correcting the links. Always displaying it in the same location would already solve the issue.