Hello,
I have an RPi4 + AMP100 (Hifiberry) with Volumio 3.378. I installed the Rotary Encoder II plugin but it looks live I have issue with my rotary encoder (KY-040 I guess).
Here is what I get with : journalctl | grep -i encoder*
Dec 10 18:26:06 litra kernel: bcm2835-codec bcm2835-codec: Loaded V4L2 encode
Dec 10 18:26:36 litra volumio[814]: info: Loading plugin "rotaryencoder2"...
Dec 10 18:26:37 litra sudo[974]: volumio : TTY=unknown ; PWD=/ ; USER=root ; COMMAND=/usr/bin/dtoverlay rotary-encoder pin_a=22 pin_b=17 relative_axis=true steps-per-period=2
Dec 10 18:26:38 litra kernel: rotary-encoder rotary@16: gray
Dec 10 18:26:38 litra kernel: rotary-encoder rotary@16: unable to get gpios: -16
Dec 10 18:26:38 litra kernel: rotary-encoder: probe of rotary@16 failed with error -16
Dec 10 18:26:39 litra volumio[814]: error: [ROTARYENCODER2] addEventHandle: stderr: /bin/cat: /dev/input/by-path/platform-rotary@16-event: No such file or directory