GTRR and QuickFind must always be followed by an IF Get(LastError) in which you take care of the situation.
In your case you can't debug the clients because they're on regular FMP, but you can set a $Var to Get(LastError) and if $Var Show custom message with the $var, halt script, end if.
post the whole script
I was about to post my script, but after putting some other error trapping in, we finally figured it out. It turns out it was my IMPORT step later in the script that was having the trouble.
Since I was connecting remotely, and the client is on their internal network, the file references were seeing the database as a new instance.
My file refernce for the import was originally this, which works for me as I connect VIA the IP Address of the compant.
Adding one line at the top, fixed it for me and for them, since FileMaker works this list in order, if the top one doesn't work for me, it moves to the next one.
Thanks for getting me to think about this more critically!