XPEnology Servers not working properly with IPv6

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I'm using my DS for a while now with IPv4 without any issues. Unfortunately I had to change my ISP for one that is using IPv6 only. This means I need to access my DS now with IPv6 only. In LAN I can for sure access it with IPv4.

THe problem currently is ,a that access from external or internal with IPv6 results in a lot of connection aborts and a very slow speed. I tried to setup another XPEnology on VirtualBox. The same slow behaviour on this machine unfortunately. I have a raspberry Pi that is working just normal with IPv6, so the issue is definetly not the router or anything else but the DiskStation.

I'm wondering if it is something based on my wrong DS settings or on XPEnology? Anyone?

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I did the same tests with a original DS216 and DS416 and both had no troubles with IPv6. It seems it is connected to XPEnology. 

Someone knows where I can report bugs/errors?

 

 

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I tried to set Synology MTU manually to 1500. After network restart MTU was on automatic again.Is it a bug? I then set it to 2000 and it persists. Speed is ok now. Issue is solved. Even when I turn it to automatic again. I guess there is something buggy....