Mike Rhodes, chief executive and founder of ConsultMyApp, said the servers were overloaded. However, he expressed surprise the Scottish Government had not left more time between the app’s launch in app stores and the day the passport scheme came into force.
The app, which was available on app stores from Thursday night, is technically required to enter any nightclubs or large events such as football matches.
Mr Rhodes said: “Fundamentally, the primary issue wasn't actually with the app per-se (surprisingly).
"The issue was actually on the servers the app communicated with simply becoming overloaded. This meant that when the app was trying to get some response from them, it was left waiting until it eventually gave up and said ‘sorry, no match’ or ‘please try again’.
"This message could, of course, have been more useful in telling the users that the service was too busy and to try again later on, but the app couldn't really progress any further at that point.”
Mr Rhodes said high demand for the app “wasn’t an unforeseen event” and was likely to get hundreds of thousands of people attempting to register at the same time.
He said: “With that in mind, even a junior developer would know they need to undertake ‘load testing’ – a software testing approach that has been around for decades to ensure that systems under stress don't collapse and simply throw errors back to the user.
"The lack of this testing shows the significant inexperience of the development team, not just in the principles of building an app, but also in the fundamentals behind software development in general.
“It can be fixed, thankfully, by doing nothing. As the issue appears to be due to an overloaded server/interface the app itself, it should start to function normally once that load has normalised.
"People should find that throughout the next day or so they will be able to finally register successfully. However, it does beg the question as to why on earth would you launch an app with so little time between it being on the app stores and it becoming essential for everyday life?”