Fortigate - Transparent - Antivirus kills connections from outside ERR_CONTENT_LENGTH_MIS

[KPS]

Hi!

I am using a FG200E-active/passive-Cluster in  "Transparent Proxy-mode".

One week ago, I did an upgrade from 5.4.3 to 5.6.3 to 5.6.5

Now, there are some issues, I cannot resolve:

If I connect from "outside" to a webserver behind the Fortigate, everything is working fine.

If I activate "AntiVirus", the webserver cannot be reached. The site starts to load, but then it does not proceed.

Sometimes, the browser-console shows: ERR_CONTENT_LENGTH_MISMATCH

Disabling AntiVirus can bypass the problem.

Do you have any idea, why this happens?

I want to avoid the downgrade.

Thank you

KPS