I was getting this error as well without the small hotel mode on. Turns out that the room parameters NOT optional as stated in the description of the field.
At least in Dutch it says the extra parameters are optional but so far I was unable to keep the system working without adding the “display_name”, “rooms”, and “persons” parameters.
Jan, can you please verify that these are mandatory and that maybe the description needs to be changed in the settings section of the plugin?