My hal file in v. 8.4 2017-07-26 caused the application to crash with "runtime error, hal stack is:Fatal: Internal value stack full". I could not figure out for 2 hours what is going on, because the syntax was OK. If I commented out the call to FindUserMailBoxName() it would not crash. If I moved the FindUserMailBoxName() to the first line after the variable declarations it would not crash there.So the problem was not in the call to FindUserMailBoxName(). The cause was in the variable declaration line: I deleted the end of this line and typed the same text again. This solved the problem. I could not see any invisible chars in this line (I turned on the "show all characters" option in the notepad++).
Any ideas why this happened?

...
string 255 mailboxcode, tstr;//the problem was somewhere here

Userr.Code="";
while (LoopMain(Userr,1,true)) begin
if (Userr.TypeOfUser==kTypeOfUserNamed) then begin
FindUserMailBoxName(Userr.Code, mailboxcode, tstr);
...