I uninstalled the addon and installed it again, without entering any API keys, the addon works flawlessly now on Kodi 18 and Estuary MOD V2 skin. No more failure messages.
Nvidia Shield TV 2015, Sony BRAVIA XBR55HX929, Yamaha Aventage RX-A3050, Dolby Atmos/DTS:X (5.1.4) Front Mains: Jamo C109, Surrounds: Jamo C103, FH/RH: Jamo C93, Center: Jamo C10 CEN and Subwoofer: Jamo J112 SUB