Hm. Perhaps I'm doing sth. wrong, but after upgrading a few sites to Drupal 8.8 I found that I cannot save newly translated nodes anymore. The error message is "Either the path '/node/[nid]' is invalid or you do not have access to it." where [nid] is of course the corresponding NID.
Steps to reproduce:
* Use sth. other than the administrator account. You should have the usual editor permissions to create nodes and create/update translations and translate any entity, of course. You should not have any url alias permissions.
* Create a new node in the original language (in my case: German)
* Save the node
* Edit the node again
* Click on translation, add translation (English)
* Try to save
=> Error.
I found that this error comes from "ValidPathConstraintValidator::validate". If I insert a return statement in the first line of that method, everything works as expected.
I don't have pathauto installed or sth. I *would* like to set/assign an automatic alias in a custom hook_node_presave, but the validation error seems to be trigged before this hook is even called. I checked pathauto and they are also using that hook to do their auto-path magic.
I don't understand the error message to be honest. Yes "/node/123" is of course not a valid alias, because it's reserved for the nodes. But I didn't assign anything. I don't even have permissions to assign any manual alias.
It used to work with 8.7 but not with 8.8.