Mosaico 2.0, Drupal 7.64 - "Sorry an error occurred and your information was not saved"

Installed Mosaico 2.0-beta4 on Drupal 7.64 and CiviCRM 5.7.2. I followed all installation instructions and installed Shoreditch then Flexmailer then Mosaico.

I’m getting an error – when I try and edit a Mosaico template and drag a block in, it gives the error message of “Sorry an error occurred and your information was not saved”.

This happens whether I upload an image into a block, or just drag an empty text box and add dummy text, or anything in between.

This also happens regardless of which base template I’m trying to edit.

If I just try and create a new mail using the base templates, I can’t preview the email as HTML or send out a test email (again, regardless of how much or how little I customise the base template) and get the same error message.

I’ve checked error logs and haven’t found anything. All modules and core up to date, any ideas on what I can do to fix this?

Thanks

EDIT: Some of the messages from Browser console. Not sure which of them are relevant:

The resource from “[default]/civicrm/css/?r=f1Bvs” was blocked due to MIME type mismatch (X-Content-Type-Options: nosniff).

.

Error while detaching the browsing context target front: Connection closed, pending request to server1.conn0.parentProcessTarget1, type detach failed

^ That one then has a “Request Stack” of around 20 lines.

Content Security Policy: The page’s settings blocked the loading of a resource at https://raw.githubusercontent.com/favicon.ico (“default-src”).

.

Loading mixed (insecure) display content “http://mosaico.io/srv/f-default/img?method=placeholder&params=166%2C160” on a secure page[Learn More] jquery.min.js:4:19318

unreachable code after return statement

EDIT:

Checked the error log in cpanel, this is an error message we’re getting at the same time:

2019-02-27 12:11:55.672900 [INFO] [203.166.241.141:58741-Q:2699717862144599633-5] File not found [/(homedirectory)/public_html/403.shtml]