InvalidArgumentException: Class "\Drupal\menu_bootstrap_icon\Form\SettingsForm" does not exist. in Drupal\Core\DependencyInjection\ClassResolver->getInstanceFromDefinition()

How did you install the module? by composer or zip? if you installed it with composer, it needs to download erusev/parsedown from the vendor (i guess you are missing this library) this library is needed to parse the Markdown startup icon.
By the way, you don't need to go to goto config to update the icons when the startup icon adds new icons, you need to update them manually or you want to change/add keywords for the icons. Then you need to go to config to update

I've also tried to reproduced this issue but nothing happened in my case .
Attaching ss for reference.

I've installed it using composer