I've come across a similar problem with sx988, but not exactly the same. I thought I'll report it here in case it is related somehow.
I use ethernet cables with a 1Gbps switch to connect the sx988 to the server (Mut@nt HD51 with DVB-T2 tuners). On the first boot once I installed OpenVix-6.4.011, the GBipBoxClient plugin launched and asked me to configure a server which I did. With the rest of the wizard choices completed the sx988 started up and BBC1 started playing. Then I noticed whenever I tried to change channel ended up showing a black screen.
If put the sx988 into standby and bring it out of standby, or if I fully reboot it, then the channel I had switched to starts streaming normally. Having to switch it off/on after switching channels to be able to watch TV made me take a second look into its configuration.
I disabled and uninstalled the GBipBoxClient plugin, then rebooted the box. I was still able to stream TV, but changing channels caused the same problem, a black screen and no streaming. I captured a log and noticed this:
Code:
< 4038.5670> [eFilePushThread] wait for driver eof timeout
< 4038.8173> [eFilePushThread] wait for driver eof timeout
< 4039.0676> [eFilePushThread] wait for driver eof timeout
< 4039.3121> [eHttpStream] openUrl failed
< 4039.3122> [eHttpStream] Thread end NO connection
< 4039.3179> [eFilePushThread] wait for driver eof timeout
< 4039.3180> [eFilePushThread] read error: Function not implemented
< 4039.5683> [eFilePushThread] wait for driver eof timeout
< 4039.5683> [eFilePushThread] read error: Function not implemented
< 4039.8186> [eFilePushThread] wait for driver eof timeout
< 4039.8186> [eFilePushThread] read error: Function not implemented
< 4040.0690> [eFilePushThread] wait for driver eof timeout
with the last two lines repeating endlessly.
Then I tried to configure the Client Mode, but it would not recognise the FTP password, perhaps because it has some symbols in it. With the FTP password the connection failed and the settings would not be saved. Without an FTP password it would not allow me to save the server IP address either. So I aborted setting a Client Mode and went to the tuner's 'Fallback Configuration', where I set up the server's IP address there. Subsequently the sx988 has been able to switch channels normally and streaming works perfectly.
I found the above confusing when compared to openATV, which uses IPBox Client and does not require FTP settings.
NOTE: I had trouble setting up NFS exports on the server. This happened because when the NFS service is installed and enabled, the /etc/exports file is created for a LAN segment of 192.168.0.0/255.255.0.0, so if your LAN happens to use a different private network block, e.g. 172.16.0.0/12, you won't be able to access recordings for playback until you manually edit /etc/exports on the server and restart the NFS service.