The most recent version of the Readeck extension moves from an API key based login to an OAuth-based login flow.
When you attempt to log in, most of the flow works, until you reach the following URL:
https://<your-readeck-domain-here>/authorize?client_id=urn%3Auuid%3A[......]&redirect_uri=chrome-extension%3A%2F%2Fjnmcpmfimecibicbojhopfkcbmkafhee%2Fcallback&scope=[...]
The below error then appears:

By doing some testing, it seems an MVP for the error reproduction is to go to:
chrome-extension://jnmcpmfimecibicbojhopfkcbmkafhee/callback (which is the callback URI), and this gives you the shown SchemeHandlerError. This suggests Orion extensions support lacks support for chrome-extension:// URIs to some extent.
Error message (for searchability):
Orion can't open this page
The operation couldn’t be completed. (Orion_RC.SchemeHandlerError error 2.)
Expected behaviour is that seen on Firefox or Chrome - login works fine, the redirect takes you to the extension showing you are signed in and connected.
Version 1.0.0 (139-rc), WebKit 623.1.8.0.0
Tahoe (26)