I'm not sure.

[nyagano]

[DenOfEquity]

Are you sure this issue is related?
Anyway, the problem is that modules/sd_samplers_kdiffusion.py, line 41, wants a definition for Heun_a_sampler, in modules/sd_samplers_extra.py. The definition doesn't exist, so loading fails.
It looks like the line has been edited from the entry for the Restart sampler:
{indent} ('Restart', sd_samplers_extra.restart_sampler, ['restart'], {'scheduler': 'karras', "second_order": True}),
so either edit it back, or delete it if there is another entry for Restart.

[Koishi-Star]

Please refrain from replacing files in the release package and follow the instructions in the README for the operation. If you're not fond of this approach, I'll swiftly release a plugin version to add this sampler as a plugin. Stay tuned for updates.