Mobile to-do list Wunderlist, bought by Microsoft for a reported $100m-$200m, is down with no immediate prospect of recovery.

The firm claims to have more than 13 million users and has blamed problems with synchronization and task creation on all platforms.

The service, when it works, runs on iPhone, iPad, Apple Watch, Android, Windows and browser.

Wunderlist first responded to growing complaints from users at 10.28 CEST (4.28 BST). Twenty four hours later, engineers were still working on a fix.

In the early hours of 22 Sept, Wunderlist could not say when it expects things to be fixed.

According to Wunderlist, the cause was a “performance issue with our tasks database.” Microsoft's firm wrote more about it here:

We're working on optimizing our database before we get it up and running again. Your data is safe—we just can't sync it to your devices right now. Some users are able to login to our webapp, however you will still not see your lists. Fear not, they are still safe. A fix is still some time away but we are making progress.

Which produced the following Tweet:

„The problem stems from a performance issue with our tasks database.“ <= This totally reads like they @wunderlist deleted the database — Bodo Tasche 🔭 (@bitboxer) September 22, 2016

With the chief executive of rival to-do list Todoist, Amir Salihefendic, chipping in:

Wunderlist has been down for the last 24h. Even if we are competitors this isn’t the way to compete. Hope it gets resolved soon .@chadfowler — Amir Salihefendic (@amix3k) September 22, 2016

Microsoft bought Wunderlist creator 6Wunderkinder in June 2015, building a portfolio of productivity apps to work on non-Microsoft mobile platforms.

At the time Microsoft reckoned the deal demonstrated its: “Commitment to delivering market leading mobile apps across the platforms and devices our customers use – for mail, calendaring, messaging, notes and now tasks.” ®