I am sure that like 90% of the People Complaining about this error have upgrade their connection to Docsis 3.1 (1 GBit Connection in Germany). This has to do with the ISP Connection and the Router used. The workaround is to set up a VPN connection, download the metadata and posters and afterwards switch back to your regular ISP connection. Take a look on this thread, there is is explained. As far as I could experience, this has nothing do to with Plex Software, only with the ISP Connection.
To sum it up:
If you have problems that metadata is slow loading and posters are only sometimes loading. Try a VPN Connection, and i promise you it will download the Posters in seconds. 
