"Could not parse time from lastsync"

Hello, I'm the admin of repo.jing.rocks (tier 2). Lately I've been seeing errors like Could not parse time from lastsync frequently on https://archlinux.org/mirrors/, but there seems to be no problem from my side: I use the provided mirror script from arch wiki, and I set up systemd.timer to email me in case it doesn't return 0. The most recent Could not parse time from lastsync happened on Jan 18-19.

My upstream is ftp.acc.umu.se, and this error also happens to them. I'm observing some other tier 1 mirrors having this error, e.g. leaseweb.net, geo.mirror.pkgbuild.com.

What could be reason? It looks like there is no user complaints about it.