Thanks again for the input.
EX9200 comes with some limitations yes. It can only be used as a MPLS edge switch (MES) For example: LSP's are not supported.
I do not use any multi-homing. Its 4 datacenters and one MX or EX in each datacenter as the core network device. Datacenters are fully standalone. Therefor i do not use any ESI configuration in any way.
I have tried to use it in all possible scenarios (single active, both active) but it does not help. In all scenario's the port is in Up/Forwarding mode (show evpn instance HOSTING-EVPN extensive) but it still does not work.
The EVPN works between the two MX'es where one is running the old firmware and the other the new. But it does not work in a MX+EX combination with old+new firmware.
Tonight we will upgrade the other MX so that both MX'es run the same version. Luckely we can do this as the EVPN part is not yet in production on the MX'es.
In the coming week i will try to find out what the best Junos version is for the EX9200 with the best EVPN support and then upgrade those. Fingers crossed!
If anyone has any information about the best software for EX9200 running EVPN, please let me know.