Dear Iuri,
I double checked your situation: I have installed a fresh instance from the 5.9.1 tarball, installed dotlrn and upgraded all packages from repositories to the last 5.9 branch versions avaliable, then I created a new community, joined it, entered the calendar UI and created a new event and the issue you described could not be reproduced.
Which Postgres version are you running? This might or might not have a role. Suggested Postgres version for 5.9.1 is at least 9.6[1]
Might help if you could provide steps to reproduce
Ciao
Antonio
[1] https://openacs.org/xowiki/openacs-compatibility-matrix