EX3300 [12.3R3.4] returns an error: remote unlock-configuration failed

Hello,

After a long break I wanted to change the port configuration on my EX3300 based Virtual Chassis.
To my surprise, the switch has returned an error (as below). I didn't change any rstp related configuration lines, only the port description.

What I tried already in order to solve that:
- run 'commit synchronize force'
- login to member 1 and run 'mgd -i' and 'commit' from master again

VC is a three member stack with JUNOS Base OS boot 12.3R3.4.

fpc0:
configuration check succeeds
fpc1:
/var/tmp/juniper.conf+.gz:1786:(8) syntax error: rstp
/var/tmp/juniper.conf+.gz:1794:(5) error recovery ignores input until this point: }
/var/tmp/juniper.conf+.gz:1797:(5) error recovery ignores input until this point: }
/var/tmp/juniper.conf+.gz:1800:(5) error recovery ignores input until this point: }
/var/tmp/juniper.conf+.gz:1815:(1) error recovery ignores input until this point: }
/var/tmp/juniper.conf+.gz:1904:(1) error recovery ignores input until this point: }
/var/tmp/juniper.conf+.gz:1907:(1) error recovery ignores input until this point: }
/var/tmp/juniper.conf+.gz:1915:(1) error recovery ignores input until this point: }
error: remote load-configuration failed on fpc1
/config/juniper.conf:1786:(8) syntax error: rstp
/config/juniper.conf:1794:(5) error recovery ignores input until this point: }
/config/juniper.conf:1797:(5) error recovery ignores input until this point: }
/config/juniper.conf:1800:(5) error recovery ignores input until this point: }
/config/juniper.conf:1815:(1) error recovery ignores input until this point: }
/config/juniper.conf:1904:(1) error recovery ignores input until this point: }
/config/juniper.conf:1907:(1) error recovery ignores input until this point: }
/config/juniper.conf:1915:(1) error recovery ignores input until this point: }
fpc0:
error: remote unlock-configuration failed on fpc1

Dawid