Hey mang, you can work around it with with some URL fiddling.
https://www.sydlexia.com/forums/search.php?search_author=Cameron
...is the successful first page (after I click your profile, then "Find all posts by Cameron").
https://www.sydlexia.com/forums/search.php?search_id=1085265506&start=25
...is the unsuccessful second page if I click Page 2. Its automatic change from search_author to search_id seems the culprit.
https://www.sydlexia.com/forums/search.php?search_author=Cameron&start=25
...is the successful second page if the URL is modified to keep search_author.
https://www.sydlexia.com/forums/search.php?search_author=Cameron&start=50
...is the successful third page if the URL start value is incremented by 25 (and so on).
Unfortunately I see no obvious way to list more than 25 posts per page (and this may be intentionally hard-coded to prevent certain abuses).
Similar fiddling also works for the main Search function, where the URL must keep mode=results (to, again, overcome the automatic change to search_id).