9.2.3 is the version my customer is using, and I have no insight into what other versions may be affected.
I have opened a case, waiting for a CR , will post it when I get it so you can watch the release notes for the fix.
In the meantime, you just need to validate any variable value you want to push into findclass. My customer coded around the issue by checking for null string and replacing with "-":
set FEHandler [findstr [HTTP::uri] "FeReqHdlg=" 10 "&"]
if { [string length $FEHandler] == 0} {
set FEHandler "-"
}
...[findclass $FEHandler $::FEserverClass]...
/deb