Sorry, I'm still in learning mode. I've read a lot, but apparently it's not enough.
I did an update of the GX board in the Multiplus II GX from firmware version 498 to 528.
The installation guide stated that the GX board will still be live in case of a soft switch off. But that wasn't the case.
The Multiplus II GX is now in the off-state and I lost the connection to the GX board.
Is there any solution to solve this problem?
Thanks for your time.
The update process consists of a couple of steps. Some questions: Did you download and save the settings file? Did you then convert it and upload it via VE.Configure Remote? From the posts, the GX device (gateway) shows being seen a minute ago while the inverter appears to be "off" in a waiting state. If you did not save/convert the config file during the update process, then take a look at the "device list" and use the VE.Config Remote to download the file, configure it and upload it. This is applicable if you use e.g. your PC and VRM to do the update. NOTE - Inverter firmware updates resets an inverter to defaults.
Thank you for your fast response.
I disconnected the Multiplus II GX from the grid. It runs on the battery at the moment. Is a connection withe the grid necessary?
Yes I have the download.
I can't convert it on the iMac.
And VE.Config Remote doesn't see the download.
It that because it only works on Windows?
In theory you can just re up load the file. Without making changes. (Depending on how far out of date it is it will work for setting up again.)
The inverter is in an off state likely waiting to be reconfigured.
Usually it is best to use an MK3 to usb and be on site, rules out the internet/connectivity as a problem especially if the power going out interrupts the router power.
You can run off the inverter, however when you connect the AC, the system will be using default settings which may not be appropriate for charging your batteries. If you used the VRM portal to update the firmware, it should have prompted you to download and save a settings file with extension rvms. Do you have this file? Uploading the existing rvsc will unlikely work and you will receive the below message. The VE.Config tools are the only way forward unless you have the rvms file someone can convert for you and you can then upload via portal. 
Thank you all for your responses.
Maybe I should give some additional background information.
I've a single fase grid house and on 25 November the installation of the inverter MultiPlus-II 48/5000/70-48 found place with a 20 kWh set of Pylontech 5000 US batteries. The inverter updated that day to the new firmware v3.12 and the GX board firmware 498.
Last Saturday I updated the inverter to v3.13 and last Sunday morning I started the firmware update to version 528. I downloaded the rvsc file in the map downloads. I read in the installation guide, that the GX board will remain active in case of soft 'switch off', so I did't expected any problem.
Everything went well till 11:43, when the system and the house lost the grid. I did unwire the inverter and rewired the house to the grid. The inverter is now running on the batteries.
Although I had read a lot already, I still have to learn more about this rather complex system.
@Alexandra
I've downloaded the 'c0619ab41a2d_inst_ttyS3_Interfaces_Mk2_Tunnel.rvsc' file. This file plain on a usb data stick put in the USB port the Remote VEConfigure is looking for the using services and doesn't find one.
Later on I will try it again on a Windows computer.
I got the settings file with extension rvsc and not rvms. (single fase?)
How do you make this one without a Windows computer?
I wil try to get one temporally. Then I can start with the VE Configuration tools.
Thank you both again. It's very helpful.
Have you tried downloading a new rvsc file using Remote VE.Config in VRM? You will still need access to a Windows computer to reconfigure it. If you have a rvsc settings file from before the MP firmware update and the latest version after the update, I can do the changes and you can upload it via Remote VE.Config.
We tested yesterday using the Remote VE.Config on a Windows laptop.
But on the VRM site, the download and upload button doesn't appear and didn't react. So it failed.
I called a Victron licensed technician and he tried with an online connection to install the firmware on the inverter. But that failed too.
So I decided to bring the inverter to the technician. I did that this morning.
He will try to set the firmware to a ESS version.
I hope he will succeed.
After that I will buy a MK3 - USB interface and hope to manage the inverter in the future.
Thank you again for your support.
