If I create a new letter or letter template, then I can only save a small amount of text (maximum three small text blocks). If I use a block of posts or more blocks, then I cannot save the letter. All settings are deleted and the window for selecting a template appears again.
Hi, bad problem. When you save the url should be changed with the newsletter ID. It means something went wrong while saving the data to the database (ask the provider to run a table analysis for your DB).
I saw that problem after a site migration where the database has been reconstructed without all the correct definitions.
You should even check the System/Logs panel for errors. You can collect more information by enabling logging on Newsletter’s main setting and setting the login level to error (or more).