Further update:
TfL have confirmed to us that this is a Mastercard issue and nothing to do with Dozens itself.
Dozens currently has a ‘shared BIN’ (Bank Identification Number) which was common with Loot who recently went out of business. We strongly suspect that our cards have been caught up in the effort to deal with the impact of Loot’s closure.
We have informed both TfL and Mastercard that our customers are being affected and we do hope they will fix this very soon.
As mentioned above, this issue is specific to TfL, and all other transactions and transfers on your Dozens cards are working normally.
To avoid this happening again in future, our roadmap includes us getting a dedicated BIN so that we are not at the mercy of external factors like this.