Yesterday C2ME released a new version that is now crashing when clicking on video settings if paired with Bobby. My best guess for why this is happening is because in this version C2ME added a new no-tick view distance.
See RelativityMC/C2ME-fabric#56 for more information as there is an issue already open on the C2ME github.

Your guess is correct, both mods do exactly the same thing: They increase the max value of Sodium's render distance slider.

Not sure what the best to resolve this is. I only recently learned that @ModifyConstant is not actually composable. I guess either (or both) mods could switch to a ModifyVariable if the method permits it (I don't actually recall what the corresponding code looks like), or we could wait for there to be a composable option for ModifyContant and then use that (and ofc make sure our overwrites actually compose), or one of them could disable their overwrite in the presence of the other.
So I'm going to wait for the opinion of C2ME's author.

In the mean time, you should be able to work around this relatively easily by opening either (but not both) mod jar and editing their "*.mixins.json" file (bobby.mixins.json or c2me.client.mixins.json) to remove the SodiumGameOptionPages entry, and then the other mod's overwrite will be applied as usual.

The most recent commit on C2ME supposedly fixes the conflict

Closing as this should be fixed as of C2ME 0.2.0+alpha5.