Bravia 2011 loose connection

Hi there,
I recently found out that the TV in my flat supports streaming, so I installed Serviio 1.4.1.2 on my Win7 PC and the TV directly found it and everything was fine. However, two days later it showed it on the list but could not connect... So I tried reboot on every device, reconnecting the LAN cable and things like that. After I reinstalled Serviio it could connect without any problem. Next day, it didnt. This is going up and down, sometimes it works, sometimes it doesnt. I have no idea which conditions are necessary for it to run.
I use a Bravio 40CX520 via a Fritz.Box 7390 (on which I dont have any access). I have a picture attached, even if the device is showed as "green" it doesnt mean that it is working...
Thanks for you help. I searched for related topics but I could find any.
Regards,
roever.

[Illico]

I suggest to clean the device list (remove button on serviio console), then save, then restart serviio service or reboot PC.

I did this already, sorry for not mentioning earlier.
Did not change anything :/

[Illico]

And do you also clean server list on Sony Network settings ?

thing is, sometimes it works just perfectly. without me doing anything.
That is the weird thing. And, yes. I also cleaned the TV list.

[NX3]

I'm sure I've read on here before people having problems with Fritz box. Maybe search on Fritz as keyword...

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