As it turns out, I messed up with one of the methods in the plugin and didn't update it as appropriate. Whilst it was apparently mildly broken in 1.16.5 (see https://hub.spigotmc.org/jira/browse/SPIGOT-6575 for all the technical jargon), this only started being problematic for 1.17, so I went through the codebase and replaced all instances of the problematic code HP had that would result in errors and even crashes. Whilst this only affected /head and /myhead, all bits and pieces that use similar codebase have been replaced.

If you are on 1.17, please update to this version ASAP.

I'd also like to use this as a opportunity to mention that some people have been bringing up performance complaints of which I've failed to reproduce or pinpoint as of so far. Unfortunately, timings have failed to give me any extra information as well, so for the time being, please use spark to report performance problems so I can pinpoint the exact cause of the lag.

Thank you in advance!