I did my first reboot after a few days of usage. I'm now having the same problem. I'll try roadam's previously mentioned solution to get a working Cinnamon back. Meanwhile, I'm still in the fallback mode.
I did use Applications / System Tools / Log File Viewer. It does show a prominent error that "/var/log/mail.log" does not exist, but it doesn't seem particularly relevant. The only other odd messages are these, which also don't seem relevant to Cinnamon not working.
mdm[1741]: PAM unable to dlopen(pam_kwallet.so): /lib/security/pam_kwallet.so: cannot open shared object file: No such file or directory
kernel: [ 1321.865069] systemd-hostnamed[3677]: Warning: nss-myhostname is not installed. Changing the local hostname might make it unresolveable. Please install nss-myhostname!
p.s. I tried the earlier commands to reinstall cinnamon then reboot. It did not solve my problem. I still have to use Fallback Mode.
And this time, I do not get any menu system at all, just desktop icons and a right-click menu.
p.p.s. One other oddity. When first launching Firefox, I get a window that says, "Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system."