This issue concerns the ‘statistics’ feature, specifically the ‘calender’ section, that shows how many cards were learned at each day. I am using the german version so if I use the wrong terminology, please let me know.
I was learning my cards late at night and when the midnight passed Anki logged the cards I learned in this session to the ‘new’ day completely. As this happened, Anki inserted a new day, at the beginning of the year and shifted all the learning progress by one day. On the newly inserted day Anki logged 0 learned cards.
Luckily I made a screenshot of the statistics page a few days before. I compared the appearance of the calendar and it seems all the learning progress has been shifted by one day.
This behavior is very unexpected, but I assume there could be an easy fix by altering the database and changing the dates there, however I was not able to find any proper instructions on doing so. Is this a known problem, and if so are there any solutions for it?
Please locate a backup that shows the behaviour in your first screenshot, then export your collection without media, share the backup and exported version online, and send me a link privately (or publicly if your data is not private)
Thank you for your reply. I played around a bit with Anki this morgning before and it seems, after a new batch of cards were due, the learned cards were logged correctly again and the extra day disappeared.