It seems that Cisco is aware of problems with the newest firmware version and specific routers. Currently not all models of the RV320 seems to have this problem, but I got the recommendation to downgrade to version 1.3.2.02.
I was not able to push and install the firmware via the SFTP Mode method described in the documentation.
I figured out version 18.104.22.168 opens debug ports for the web interface, which aren't effected by the protocol issue. Just open the web interface through port 8008 and downgrade as usual to the stable software.
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