The error "This block contains unexpected or invalid content" is shown in the block editor when a blocks formatting doesn't match what the current version of the block editor expects.
On core WordPress blocks, most of the time, this is caused by WordPress changing how they handle things and simply not caring about the end user experience and simply being willing to break things.
This is why we tell people not to play with early/experimental features that are in development. Unfortunately, the block editor is "mature" and shouldn't be breaking things, but it still does.
Identify the source of the block
The block will either be a WordPress block or a third party plugin block.
If it's a core WordPress block, you're just out of luck and will have to tolerate that they break things.
If it's a third party plugin block, contact them for support and guidance, and consider not using third party plugins for additional blocks, if you value maintainability.
An exception to the above is plugins (such as Grow or Tasty Pins) that alter core WordPress block output. Disabling or removing these plugins can cause the block editor to no longer recognize the markup they previously added.
What to do
In most cases, simply clicking "resolve errors" will fix it.
Somtimes it won't be able to and will generate a custom HTML block with the previous code and you'll need to figure out what to do with it.
Does it need to be fixed?
In most cases, the existing post will be fine - just check the front end. If that's the case, you don't need to go in and change or update those blocks unless you need to do a bigger update on that post in general. But fixing the block error will only make the block editor happy and not necessarily do anything on the front end.
This is not theme/Feast related
We go out of our way to avoid breaking things in this way, and to date, haven't had any of this error pop up for any Feast blocks.