As @sourcejedi commented I tried
systemctl --failed and got the following:
UNIT LOAD ACTIVE SUB DESCRIPTION
* media-win-Data.mount loaded failed failed /media/win/Data
* media-win-Misc.mount loaded failed failed /media/win/Misc
Which are Windows drives (without OS installed on them). After running
mount -a I got the message that
/dev/sda5 cannot be mounted because:
The NTFS partition is in an unsafe state. Please resume and shutdown windows fully (no hibernation or fast restarting)
I turned off
fast startup (recommended) in Windows 10 according to Unable to mount Windows (NTFS) filesystem due to hibernation. After shutting down Windows, Ubuntu boots without complaints.
Interestingly, I was using
fast startup option for shutting down Windows for more than a month and Ubuntu has not complained about it before. That's why I didn't think that this could be the reason.