I’ve got a large database (170k units) and I started getting synchronization timeout.
I reduced the database size to about 90k.
Performed database checking (at some point it kept failing, but now it succeeds).
Performed the other checks (empty cards, file check with removing files from the bin).
The progress bar in “sending to AnkiWeb” progresses further, but it is still timeout 1/3.
The collection size: