Sentinel-2 is a brand new and complex system, it is not a full surprise if it is experiencing some glitches. ESA is publishing weekly status reports, and the last two ones quote an issue with the mass memory. From weekly report #11, 24 Oct – 30 Oct 2015:
The spacecraft has experienced this week an anomaly of the Mass Memory and Formatting Unit (MMFU), which is currently under investigation with the full support of the satellite prime contractor and the MMFU manufacturer
This anomaly was not yet solved at the end of last week, as said in weekly report #12 : 31 Oct–6 Nov 2015 :
The anomaly of the Mass Memory and Formatting Unit (MMFU) experienced since last week still persists. Analysis is ongoing with the full support of the satellite prime contractor and the MMFU manufacturer. An MMFU application software fix is being prepared and trouble-shooted on Engineering Functional Model and the Sentinel-2B platform.
I do not have much more information, but if the issue can be solved by a software modification, it means that all the equipments are fine. The ESA teams do not seem to be worried, as the outlook is :
Resolution of the MMFU anomaly and return to nominal S2A ramp-up activities
However, the release of Sentinel-2A preliminary product seems to be postponed a little, as the outlook also tells :
Opening of S2A core products access to all users –pending resolution of the ongoing on-board anomaly
To get more information here is where the status reports are provided, and we should thank ESA for sharing the information in such an open manner.