Bad response code for POST https://xxxxxxx/seafhttp/repo/head-commits-multi/: 500

on 19.1, I upgrade on manually from seafile windows client 6.1.1 auf 6.1.4
(manually as the update functionality did not work)
since then the seafile.log containes every 30 seconds the follwing entry:
[01/22/18 21:27:41] http-tx-mgr.c(2304): Bad response code for POST https://xxxxxxx/seafhttp/repo/head-commits-multi/: 500

syncing itself seems to work also the web interface

what can be the cause of this?

edit:
I reverted to 6.1.1. the errors in the logs are gone, so it seems to be related to some changes between 6.1.1. and 6.1.4

the code line throwing the error is related to this commit, introduced with 6.1.4