Thank you for the tip. I tied the shell script and it really worked. The update is ok now - I am at 9.0.0 now.
But - this cannot be the real solution? There must be a bug somewhere.
Does anyone now if it is possible to upgrade to 9.0.0. the "legal" way?
I updated my WETEK-Play from LE to 8.95.1. without any problem.
Then the update to 8.95.2 and the following Beta Versions were not possible: In each case I got the message "missing (target) kernel.img or SYSTEM".
Now I tried to update to 9.0.0 and I get the same message 
What did I do: I tried to update in three different ways:
1. directly in LE
2. Download of the LibreELEC-WeTek_Play.arm-9.0.0.tar - File to the PC, then copy it to the update-folder on the WETEK
3. same as 2., but unpacked the img before copying to the update folder of the WETEK.
Same result in all three ways: No update. Instead the message "missing (target) kernel.img or SYSTEM" and then starting the v8.95.1.
Can anyone tell me how to update the WETEK to 9.0.0.?
Thanks in advance
