Hope it helps

First of all, I'd like to thank you for your contribution and your efforts. Unfortunately, I can't do much with this information. Where do I need to make which buttons and actions visible again? Can you tell me what the fields are called? Is this even possible via the Studio? It's so frustrating how much can be broken with an update, as happened in version 19. 

I also don't understand why karma points are needed to properly reply to threads or to mark a self-created thread as solved. The way this is being handled is so nonsensical. Nevertheless, thank you to everyone who takes the time and offers support.

Because the Pause / Resume buttons have been removed from the Project Task form view in timesheet_grid enterprise module.

So you can either add back the buttons + invisible fields to trigger state reload, or if you do not have access to do that, you can call manually the actions through ORM calls via browser console (use action_timer_resume if you want to resume):

odoo.__WOWL_DEBUG__.root.env.services.orm.call('project.task', 'action_timer_pause', [odoo.__WOWL_DEBUG__.root.actionService.currentController.currentState.resId]).then(() => odoo.__WOWL_DEBUG__.root.env.services.action.loadState());