If WiFiman worked and does not work anymore after the firmware upgrade or by a time you need to generate new java certificate.

If you access https://unifi:8900/proxy you should see following:

x509: certificate has expired or is not yet valid

Or

Unknown certificate authority

To fix the problem proceed with following commands issued over SSH on UDM device:

unifi-os shell
mv /usr/lib/unifi/data/keystore /usr/lib/unifi/data/keystore.backup
systemctl restart unifi

Finally – re-enable WiFiman in Unifi network console. If you will visit now https://unifi:8900/proxy you should see WiFiman statistics in JSON format and you will be able to run Signal Mapper from WiFiman app.

More information can be found here.