Error connecting Jetpack to WordPress.com: Error HTTP 503

While writing a blog post, there was a notification to refresh the connection to LinkedIn. When trying to refresh the LinkedIn connection, I got this error:
Something which should never happen, happened. Sorry about that. If you try again, maybe it will work.
Error code: -32300

I found this old post, recommending to disconnect Jetpack from WordPress.com:https://wordpress.org/support/topic/jetpack-sharing-to-linkedin-not-working-error-32300/

I did this, and then clicked to reconnect Jetpack. I then get this error every time I try:
Error Details: The Jetpack server was unable to communicate with your site https://www.chalkandmoss.com [HTTP 503]. Ask your web host if they allow connections from WordPress.com. If you need further assistance, contact Jetpack Support: http://jetpack.com/support/

I’ve followed the guide here:
https://jetpack.com/support/getting-started-with-jetpack/troubleshooting-tips/
I disabled all plugins, reactivated Jetpack plugin and tried unsuccessfully to reconnect Jetpack to WordPress.com. I then activated plugins and still couldn’t connect Jetpack.
As per step 4, I checked XMLRPC file and it displays as it should, just on one line.

I followed step 7, entering my URL into the debugger:
https://jetpack.com/support/debug/

There I get this error:
XML-RPC is not responding correctly ( 503 )
It looks like XML-RPC is not responding correctly. Please make sure XML-RPC is turned on for your site and is set up to respond to all content types. You can test this yourself by running the following command from the command line
(I don’t know how to do this)

I’m on WordPress 4.9 (my theme hasn’t been updated for a year, so I imagine it’s incompatible with 5.0 and beyond. I’m going to switch soon). I’m on Woocommerce 3.5.2.

I’m afraid I’m not a developer, so please be kind!

Thanks everso

Anna

The page I need help with: [log in to see the link]