I just tested an expression with just over 2000 characters. No problem. Over 5000, still no problem. Went to just over 50000, and my QlikView crashed out when I clicked OK for the expression. 20000 ran OK. 35000 ran OK. And 50000 crashed again when I clicked OK.
Crashing out is obviously not an intended "maximum expression size error". So I would conclude that there isn't supposed to be a limit, but there IS a bug. Now, I was testing with version 8.50.6231.5. It would not be surprising if the bug had been fixed, or even made more severe in a later version. It would not be surprising if the bug were intermittent, or specific to my hardware, or aggrevated by me not having rebooted for several days. It might not have anything to do with QlikView at all.
What are you actually seeing that makes you think there's a limit at about 2000 characters? We should probably start there.