Dear @NateWr @Gecuimar and others
I have the same problem. See also my post: [Solved] Problems with api submissions error on submissions page
I guess it is an error in the code. None of my changes in .htaccess files works!
An answer to @NateWr 's question:
I tried removing the double “/” and that does not work!
This is the link that OJS tries to access:
https://domain.sub.se//jounralX/api/v1/_submissions?status=1&searchPhrase=&count=20&offset=0&_=1550044118374
→ causes 500 Internal Server Error 
This:
https://domain.sub.se/jounralX/api/v1/_submissions?status=1&searchPhrase=&count=20&offset=0&_=1550044118374
→ causes 500 Internal Server Error
However only:
https://domain.sub.se/index.php/jounralX/api/v1/_submissions?status=1&searchPhrase=&count=20&offset=0&_=1550044118374
→ causes 200 OK!