Hey @manthankanani we have released Boost version 3.10.3, which should resolve the Error: Maximum call stack size exceeded Manual Critical CSS Loading issue. Please ensure that your browser cache is refreshed after updating the plugin, and before you generate the manual Critical CSS.
Please let us know if you encounter further issues, thank you!
We have released Jetpack Boost 3.10.3 that should address further issues reported relating to manual Critical CSS generation.