Err Content Length mismatch???

What version of BIGIP is this happening on? Have you taken a tcpdump and reviewed the HTTP Request to ensure content length matches with the actual data?

tcpdump -i 0.0:nnn -w /var/tmp/xyz.pcap host and host

Do you see any errors in /var/log/ltm and /var/log/asm at the time of the request?

JG2017 have you tried adding response chunking in http profile to rechunck or selective. Regarding what is causing mismatch. you need to capture traffic using http watch or fiddler.

You can write irule below and check.

when HTTP_RESPONSE { HTTP::disable }

map this to vs .