When going to almost any search related page in Central Admin or SSP you get plain white screen with black text saying: "Unknown error"
This one falls probably in the category 'many things can cause this', but in my case I had modified the application.master because of branding.
Luckily in this scenario the Central Admin site is located on a separate application server (indexing), so best solution was to revert the application.master into default OOB version only on that server. No end-users will access sites on that server, so I consider that a safe workaround.
In cases where you really need to modify the application.master, there comes other solutions, such as presented here.