Forge 1.20.1 (Tested on both 47.2.17 and 47.2.21)
Placebo v8.6.1
Apotheosis v7.3.3

I removed all other mods from my modpack other than just Apotheosis and Placebo, and I get the following error (it doesn't appear in debug or latest.log, only when I use the game output window)
Caused by: java.lang.RuntimeException: java.lang.IncompatibleClassChangeError: class dev.shadowsoffire.placebo.color.GradientColor cannot inherit from final class net.minecraft.network.chat.TextColor

Here's my debug.log and my latest.log

Edit: Confirmed this occurs also with Placebo 8.6.0 and 8.5.9

This (annoyingly) happens when Access Transformers fail to apply, which happens when there is a dependency error. It should be near the top of your log file.

If you have just those two mods you would be missing the required dependency Apothic Attributes.

ah thank you much! that did it

ah. good to know.