"I first expand it to 16384 in the proxy action"
What is "it" that you expand?
"I then switch off the option "Set the maximum URL path length to " and the error is still there."
Maximum URL path length and header line length are not the same and are in different parts of the settings.
Shouldn't you be making the changes in HTTP Proxy Action > HTTP Response > General Settings > "Set the maximum line length to"?
My "Set the maximum line length to" in that area is 7000 and "Set the maximum total length to" is unchecked. I have no problems loading the web page.