@Interstellar 

Such things unfortunately happen a little too often. there are very often problems when there is an update especially with routers.

but ok, I think your only option is to do a factory reset and readopt the router,

  @MR.S Thank you for your reply!
 I have to unfortunately concur with you on the “little to often” part, especially in conjunction with the restrictive replacement policy I'll be tempted to start looking into alternatives if this will end up creating significant Downtime.  

Regarding resetting the Router & Readopting:
The Site has a lot of incoming VPN Connections, some of which are older Open VPN Profiles, back I migrated the Site from a defective ER605 V1 to this V2 those were somehow bricked (I then assumed because of the different Hardware) will a Reset do the same?

  @Interstellar 

if you reset the router, you will not lose any configuration, but you must take a backup of the controller anyway, always take a backup :-)

reset the router with the reset button, then delete the router from the controller before adopting again, all the old settings will remain.

backup first :-)

  @Interstellar 

hmm, that was weird. it shall not happen. I have re-adopted routers many times, very many times due to problems after updating, but everything has worked as it should afterwards. but I have used the Windows controller, there shouldn't be any difference.