There was congestion situation when some of the requests went through, but some did not and at some point card payments did not work at all. This was when the system was taken down to make changes. Banks urged customers to equip themselves with cash.

The company managing terminals Nets Estonia has confirmed that the failure resulting in overloaded card payments was caused by old data erasure process which has become too bulky. In the coming night the maintenance operation will take place which is expected to eliminate the problem permanently.

Comment from EISA:

According to head of EISA Vital Services Protection Division Urmo Sutermäe, Nets Estonia is not itself a vital service, but if their services are disrupted for any reason, it prevents banks to offer a vital service. In his opinion, it would help to reduce such disruptions in the future, if services continuously evaluate the extent of cross-dependencies and their impact and reduce the associated risks by having alternative solutions.

Not clear what merchants should do. Should they have a backup PoS terminal serviced by different card payment processor?

Links:
http://tarbija24.postimees.ee/3196937/ria-kaardimaksetega-seotud-riske-saab-vahendada
http://uudised.err.ee/v/majandus/61eb0173-1457-4edf-b4f7-c5aca3c7cc5a