At least from 1086 up to 1368.
Through all devise that will be manually initialized.
Device mode start - Manual.
Normal beta or with PADEMU
Haven't tried 0.9.3.
When I will set custom THM, my device will be manually initialized
("USB device mode start - Manual), save setting, reboot PS2.
Launch once again OPL, start any device, OPL will crash:

More info about this issue can be found here:
https://www.psx-place.com/threads/open-ps2-loader-v0-9-3.13415/page-41#post-202250.
So you have a theme on USB, have the path saved and it crashes regardless which device you are starting manually, or just on USB?
I know you mentioned 'start any device', but I just wanted to make sure there's no misunderstanding on that.
It crashes when you start the device the theme was saved from.
I found another similar / related issue.
Start OPL with HDD and USB set to Manual
Start one of the devices
Load a theme from that device but don't save cfg
Start the other device
OPL will crash.
If you save cfg before starting the second device it doesn't crash.
Tested with r1368 compiled by AKuHAK's bot and r1108 so it looks like it's always been that way.
THX for the confirmation that this issue happens on the device the theme is saved on (which points to another cause, then if it would happen 'globally' IMO) + your other tests and bug-find.
Problem should be fixed, more info here:
https://github.com/ifcaro/Open-PS2-Loader/pull/217/commits/ab0319ce0273827f4f36e88ea52ad32b26210fd7,
https://www.psx-place.com/threads/open-ps2-loader-v0-9-3.13415/page-42#post-204419.
Most helpful comment
THX for the confirmation that this issue happens on the device the theme is saved on (which points to another cause, then if it would happen 'globally' IMO) + your other tests and bug-find.