NHS Greater Glasgow and Clyde - Scotland's largest health board - has made an "unreserved apology" over an IT problem that led to over 700 patient appointments being cancelled.
But mystery surrounds the failure, as first it was blamed on a hardware problem, and then a software glitch with Microsoft Active Directory.
The problem affected around ten 10 hospitals across the health board's area. A server crash and the failure of a back-up system was originally said to have caused the chaos - leaving clinicians unable to access patient records or clinical images, for instance.
Both outpatient and surgery appointments were axed as a result, including cancer treatments.
NHS Greater Glasgow and Clyde chief executive Robert Calderwood said, "I apologise unreservedly for the inconvenience this has caused to our patients, who have had their procedures postponed.
"I will ensure that everything possible is done to get their treatment carried out at the earliest possible opportunity."
Yesterday (Wednesday) late afternoon the health board said, "Despite round the clock efforts by our own IT specialists and our international suppliers, the IT problem affecting clinical and administration systems is not yet fully resolved.
"However, at lunchtime today we were able to re-establish a basic level of IT service to enable some of the clinical services affected to resume normally. Further work will be carried out during the afternoon to ensure that this recovery is sustainable and that full IT functionality is restored."
It added, "All patients with scheduled appointments tomorrow (Thursday)should attend as normal as it is important that they are individually clinically assessed despite the current disruption."
But the problem now seems to be fully fixed, after IT suppliers Microsoft and Charteris were called in.
Calderwood told The Scotsman, "As of 2am this morning (Thursday) our technical team, supported by Microsoft engineers, have re-run and re-profiled all of the IT systems that were affected and they are all operational as we speak."
He added, "Clearly, as the hospitals and departments busy up and more and more users come onto the system, that will be the test, but as we stand at the moment all systems are operational."
Calderwood told The Scotsman the problem was caused when a "key programme on the computer system became corrupted over the weekend".
"This was a corruption of a software programme, which therefore moved from server to server," Calderwood said.
He told The Scotsman, "About 99 percent of all IT networks across the UK are built on this programme, called Microsoft Active Directory, and Microsoft we have been working with for over ten years, and this is the first time we have had a fault. Microsoft themselves say they have never had such a fault reported across many years in all of their business areas."
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