After upgrading SG135 to FW 9.711-5 our RED 50 connection is lost.

Hello spechto,

Thank you for reaching out to the community, you have delete the red config and add it again, and the check the logs under the /var/logs - red.log !!

I found this in the actual log:

2022:09:29-09:51:44 utm01 red_server[21222]: SELF: Cannot do SSL handshake on socket accept from 'xxx.xxx.xxx.xxx': SSL connect accept failed because of handshake problems
2022:09:29-09:51:46 utm01 red_server[21223]: SELF: New connection from xxx.xxx.xxx.xxx with ID xxxxxxxxxxx (cipher AES256-GCM-SHA384), rev1<30>Sep 29 09:51:46 red_server[21223]: xxxxxxxxxxxxxxx: Device config was not yet uploaded with the current firmware version '1-501-bb7bd1013-0000000'
2022:09:29-09:51:46 utm01 red_server[21223]: xxxxxxxxxxx: Connection is refused as device config was not yet uploaded.
2022:09:29-09:51:46 utm01 red_server[21223]: xxxxxxxxxxx: Sending json message {"data":{},"type":"DEVICE_CONFIG_NOT_YET_UPLOADED_TO_PROV"}

Perform under the cli the following: 
telnet <Public IP connected on the RED> 3400 
telnet red.astaro.com 3400 
see if it is able to connect successfully or not ?

I have tried the variant "Delete RED and register again". And the RED50 is back online. Thanks for the great tips!

you're welcome spechto please vote up the verify button !! 

you're welcome spechto please vote up the verify button !!