Can you reproduce the issue on our demo instance (https://demo.sogo.nu/)?

I can't reproduce the issue on the demo instance. Could this be related to mailbox hierarchy?

SOGoDraftsFolderName = "INBOX/Drafts";
SOGoSentFolderName = "INBOX/Sent";
SOGoTrashFolderName = "INBOX/Trash";
SOGoJunkFolderName = "INBOX/Junk";
NGImap4ConnectionStringSeparator = ".";

I've tested with the dot separator and it was still working properly.

Do you have any error in the console of the parent window?

We have this/ a very similar error now too, I hope it's ok that I hop in this thread. I updated the system from 5.0.1 to 5.3.0 and since then the write-new-mail-window loads endlessly. I get a slightly different error:

stateService.ts:540 Transition Rejection($id: 0 type: 6, message: The transition errored, detail: TypeError: Cannot read properties of undefined (reading '$account'))
_defaultErrorHandler @ stateService.ts:540
stateService.ts:541 TypeError: Cannot read properties of undefined (reading '$account')
at r.$formatFullAddresses (Message.service.js:214)
at r.init (Message.service.js:128)
at new r (Message.service.js:26)
at Account.service.js:481
at angular.js:18075
at m.$digest (angular.js:19242)
at m.$apply (angular.js:19630)
at k (angular.js:13473)
at v (angular.js:13730)
at XMLHttpRequest.y.onload (angular.js:13635)
_defaultErrorHandler @ stateService.ts:541

In our installation it seems that something goes wrong with $account but the result is the same. Using the modal/overlay instead of the new window still works but it would be nice if you could give us a hint where to search for possible reasons.

Thank you!

Nothing is logged for the parent window.

My assumption about mailbox hierarchy is correct. The new message window loads normally with SOGoDraftsFolderName incorrectly set to "Drafts". (This of course also results in a broken drafts folder.)

As of 20211214 nightly build the new message window is working again, but the behaviour regarding identity selection changed. Previously all identities derived from LDAP (multiple mail attributes) could be selected. Now only the first is shown. This is still working as expected in the regular new message dialog.