I have a dataset of CDS spreads for European sovereign from Thomson Reuters Datastream. I noticed for some entities in some timeframes, spreads are essentially stale. For example in the case of greece roughly in the timeframe between 2012-2017 spreads were at exactly 14904,36 bps. What is the precise reason for this? I am assuming that CDS trading for Greece was banned during that timeframe or banks weren't willing to issue CDS contracts. Does anyone have any idea what exactly the reason was? Preferably with a citable source.