This has actually happened to me in the past too but… I could never reproduce it afterwards. The note about having to select the date from the calendar is highly suspicious, let me ask the web team about it.
In a past job we had a maddening UI bug which the end users could reproduce it on demand but when you put a developer in front of the software, it never happened. Turned out that it was a bug that was specific to using the mouse to select from a dropdown; of course the developer knew the interface inside out and was using the keyboard shortcuts all the time… It took days to work out what was happening. I wonder if this is the same sort of thing.
(I’m also reminded of https://www.reddit.com/r/talesfromtechsupport/comments/3v52pw/i_cant_log_in_when_i_stand_up/)