I am guessing, but I am thinking that one of the MySQL reserved words is not compatible with the database you are using. I am not posting the name of the database but I had the same error in a YouNet app. YouNet provided me with a more compatible SQL statement for Advance Blogs and it worked perfectly.
I think PhpFox needs to check this since this database is one of the most popular open-source relational databases out there.
JohnJr - can you please verify the following as it does not seem to follow sequence - meaning there is no similar value sequence for monthly, yearly, annual...
1. Regarding "admin can select whichever custom fields they want to be available as additional filters"
==> You can create custom fields for each user group and tick on the option [Include on Search User]. In this way, you can configure to support advanced filters for paying subscribers only.
2. Regarding Filter block should open in the right/left: we agreed with Scheinwelt-Media's suggestion.
3. Regarding separate for Basic filter and Advanced filter like your screenshot: PHPFox has not supported yet. You can add small custom work for this case.