~~~UPDATE: The issue has now been resolved and transactions between the regions mentioned below are now functioning as before!~~~
Bricklink have issued a PSA (public service announcement) which states that one of their partners has experienced a cyber incident on a number of their systems and have blocked data passing between them both, to ensure security is maintained for its users.
Blocking transactions between two certain regions, non-EU sellers to EU buyers and Non-UK sellers to UK buyers.
This comes just days after Brickset went offline for a number of hours. The two are most likely unrelated but until Bricklink update us as to what has happened we can only speculate.
For the time being please watch this space for updates on the matter. Bricklink have given a date of no later than the 16th March, so hang tight!