Issue: Cant get past step 21: import edits

What happened in automatic mode? If you saw no errors, and if you have a large number of edits, it might have taken more than an hour which would have resulted in your admin account automatically logging out. You just need to navigate back to the importer through the admin panel, and it should continue in automatic mode from whatever edit it did last.

I would not recommend using manual mode on a large wiki. If you have 100,000 edits you will have to click through at least 200 times to get past step 21.

I dont know what exactly happened, as it just froze. No error in the log. The only option was to go to manual mode. But then it would no longer go back to automatic mode.

Today I tried again and it started at the start and then went straight through to step 21 like you said. Step 21 took 2 seconds. Weird!

Leaving this open until the situation I mentioned is resolved. It is probably what happened in your case:

Every 1 hour XenForo tries to update the CSRF token. It does this asynchronously on regular forum pages, but the standard XenForo JS is not loaded during install/upgrade/import. Here, it is possible for the token to become stale, especially when a regular forum page is open in another tab -- that forum page is able to update the token behind-the-back of the importer. As a result, the install/upgrade/import mysteriously halts when it attempts the next batch due to an unseen token mismatch error. Refreshing the page usually allows the user to continue as if nothing happened, because the token has been updated by the reload.

Marked this as fixed in the next release.

Each install/upgrade/import batch will now update the CSRF token used by the window to initiate the next batch. Additionally, support has been added to update the CSRF token in wiki Javascripts if XenForo has updated the token via another script in the same window. This prevents issues like the activity feed not working after 50 minutes, or the categories field not working if you have been editing the same page for the last 50 minutes.

This is less of an issue in vBulletin, because the CSRF timeout is 3 hours compared to XenForo's 1 hour, but we have added an asynchronous token updater to be consistent with the XenForo behavior here; the CSRF token is now automatically updated every 150 minutes.

I am probably getting the same. Import started quickly until Importing wiki pages. I see 2 listings of:
'Step 14
Importing wiki pages'
And then it just keeps getting stuck. Once I am in manual mode, it will not go back to auto mode.

What does the network activity reveal?

Does the last request finish? If so, what is the output HTML like?
If the last request does not finish, does the browser report some cryptic error such as ERR_INCOMPLETE_CHUNKED_ENCODING? PHP logs might tell you if there is a timeout occurring.

I have also had problems where a server firewall might temporarily IP-ban the user running the importer for some reason. This was a server configuration issue; usually waiting for the temporary ban to end or manually whitelisting the IP allowed the user to complete the import.

If you have legacy syntax support for MediaWiki, there is a known issue where nesting certain specific tags will trigger an infinite loop. This issue was only documented in vBulletin, but it might also be possible in XenForo. If this issue is relevant to you, you should be able to complete the import if you disable MediaWiki syntax and try again.

This seems to be the cause:
https://www.vaultwiki.org/issue.php?issueid=4953

I doubt an E_NOTICE would have caused the importer to halt, unless the importer was running in debug mode. You should have seen the message in the progress window in that case. Also, if debug mode was on, you would not have a large number of those errors in the logs; instead you would only have 1 message for every time you refreshed the importer. There is probably another issue that is halting the importer.

Unfortunately it is not resolved by the above fix. The issue remains.
I can process a batch manually. When clicking auto mode, I get a warning that the request did not complete.
I tried to get more information by inspecting the page and randomly trying manual and auto mode. It did not work for some time.

The console did log a get /vault/index.html request that resulted in a 404 not found.

Suddenly, it went back to the start screen. I then started the process again and it went into auto mode.

The link to auto-mode is a direct link back to the start screen. There should be no Javascript listeners on it that would make the link not follow, and there is no processing done at the target location that should make it take a long time to send a response (you have been at the start location before). What do you mean by "a warning that the request did not complete"? Is this a browser-level warning, or something output by XenForo?

If you still cannot complete the import, and you are unable to locate the cause of this warning, we can investigate through Ticket Support.