Not exactly sure what happened with the search functionality in the opensource software that I've been working with. After going through the php and template files line by line, nothing ever jumped out as being a problem. I even tried reverting to an older version of the search code + template, but that didn't work, either, which really doesn't make sense. I finally did get everything fixed, mostly through a painstaking process. I've not had this problem before but I can now see what happened (I think): First when I downloaded and unpacked the upgraded version, I had one file (the search template!) which was empty (mistake no. 1). Not sure how that happened, but I think this begin the problem. I uploaded an empty file, which then started the chain reaction.
Then when I upgraded (attempt no 1) I just used the built in upgrade script, so I was kind of working off of old data and old smarty (mistake no. 2). Although this project relies on smarty templating for the php. I've never felt comfortable working in smarty. I feel much better going directly to the php -- in this case, I think was mistake no. 3.
Although I cleaned out my web caches, I'm not sure I ever cleaned out my smarty cache (mistake no. 4). To be honest, I didn't even consider that and under normal circumstances, it's probably not a problem, anyhow.
The end solution was to delete all of the content in the web folder (actually I put an blank index page with a note about upgrading) , dump the database, dump smarty, and delete all caches.
Of course, since I had backups of everything, once I installed clean copies from a new download (doublechecking to make sure all of the files were as they should be) then I reloaded my database tables (mysql) in one by one, then verifying in the database that, yes indeed, they did import correctly. Once all of that was completed, I doublechecked to make sure it was all working correctly, and sure enough, the problem was solved!
Final step of course, was to take the site back online and THAT is the end of that story. I hope.
Sounds like a fun weekend and past few days, doesn't it?