Then, I migrated emails to a newer account with the app. Here are some screenshots about it.
There is no account. What to do first is add an account.
Click "Add account" at top-right corner. Verification will be required.
Now ready for exporting/importing emails in Protonmail.
With an older account, I exported emails as
.eml is also supported.
With a newer account, I imported them. I chose "Import local files":
Besides, if you have multiple accounts both of whose subscriptions are greater than free, you can directly "Import from account".
.mbox files exported beforehand:
Thus, I recreated my account and succeeded in migrating emails.