I don't get this behaviour. If I explicitly type sprintf(proto)() in the search box, slick throws away the spurious trailing parentheses and does the correct search. If I type sprintfx(proto)() in the search box, slick says "Tag 'sprintfx(proto)' not found" i.e. it's thrown away the extra parentheses.
Does find references work for you at all? e.g. if you hit Ctrl+/ (push-ref) does it find references to the symbol the cursor is on?
Did you install the latest hotfixes, making sure to install the latest "required" hotfix before the cumulative hotfix?
There's some things you could try...
1: Try rebuilding the tag files involved - the project/workspce tag file and the C/C++ library file.
or
2: Try a brand new config directory. Create a new empty folder called e.g. c:\temp\slick_config. When you invoke slick, add -sc c:\temp\slick_config to the command line that invokes slick. Slick should create a new config folder named 13.0.0 in the slick_config folder. You'll need to click yes when slick prompts you to create tag files for C/C++. Without doing any other customisation, open the project you were on and try the sprintf find references problem. If the problem is gone you might want to consider continuing with the brand new config folder. You could try importing some of your config from you old config folder - especially key bindings - use "export/import" in the key bindings dialog. There's some other files that would be safe to copy over. If you copy vusrdefs.e over, after loading it (type its name inclusing full path on the cmd line an dpress return) you should immediately check if the find references problem has come back.
or
3: In tagrefs.e line 484, you'll see this line
if (find_refs(search_string,1)==0) {
You could insert a debug line immediately before this
say('Refs debug :' :+ search_string);
The say command creates a debug window and shows the message - see what it thinks search_string is.
Graeme