please provide more steps…  are you going to site.com/forum/members? and then entering a search string?  any other settings on that form?

what are you searching a member for?  by that, I mean are you searching on the display name?

what was on the page before searching?  the complete member list?

any errors after searching reported on forum – toolbox – error log??  or in the php error log?

the search is of course done with query args… do you have any kind of security plugin or server side that might be blocking query args?  such as mod_security…

When I click search, the page just reloads

so same if you search for “Lisa” vice a first and last name?  suspect so, but worth checking…

and answers to the rest of the questions?  need some info to help debug your local issue…