Disconnect other BDM-Devices. Move OPL to mc0:/APPS.
It looks like not only USB but FireWire and MX4SIO are considered BD devices, all of these can be run using UDPBD or not and UDPBD doesn't have a setting for itself on the UI.
I only have this (external HDD) BD device connected to my PC and I'll try with OPL in mc0:/APPS since it's in mc1:/APPS but I want it to be in the latter so that my game saves are saved on a non-FMCB MC. Also, UDPBD works without setting a static IP address on the PS2 unlike what you state since it works with DHCP and also unlike what you state, it's possible to use UDPBD using a router since YouTube videos prove it.
What are BDM-devices? I thought they were called BD.
I forgot to mention that if using SMB, the first level's last FMV and the second level's first FMV on The Incredibles - Rise of the Underminer (PAL) have no sound if I store the game on an external HDD connected to the PC but if I store it in the PC's HDD, both have sound but one of the two FMVs has a strange whistling noise.
EDIT: Is it possible to update OPL from within without having to manually download each new version on the PC (and transfer it to the PS2's MC)? And what about games' settings? What do the "USB prefix path", "ETH prefix path", "Automatic HDD spin down" and "Start NBD Server" settings do? UDPBD server doesn't work on PowerShell.
EDIT 2: I just tested UDPBD with OPL in mc0:/APPS and it worked, but like I stated, I want OPL to be in mc1:/APPS so that my game saves are saved on a non-FMCB MC.
EDIT 3: UDPBD sometimes doesn't show games until I exit OPL and/or close the UDPBD server and run the former and/or the latter again.