This is becoming more of a feature enhancement request than a plain bug report Heading for 2.6.1 or 2.6.2

Keeping the last 2 or 3 backups is probably all you need. And even taking an hour for a BIG backup isn't bad. (YaBBForum used to take several hours on its old host.) So if you can use cPanel for the backups, you should proabably do it that way.  (But pointing out the XMLHttpRequest process being used by cPanel might point YaBB in a better direction for backing up.  )

If the other archiving methods didn't work then I strongly suspect the problem is not with YaBB's backup but with how the server is configured to handle archiving. Not having access to the host is going to make it tough as I think this is something that has to be looked at by the hosting service. My best guess is there's some sort of limit you're hitting - memory, CPU or a software setting - that's stopping the zip at 13MB. You may need to ask the site owner to make sure there are site backups being made on the server and that those are being downloaded for safekeeping.

(I know of one shared hosting service - not a cheap one, either - that offers free site backups but they don't tell their clients that those backups are only saved for 12 hours unless you pay them big money to email them to you - or you go online every 12 hours to grab the newest backup before they delete it.  )

Do you have access to tar? Do you have access to any server backup utilities?
It may simply be that zip running through YaBB on that server can't handle that big of a folder if it's timing out at 13MB.

You might also want to post this on YaBBForum so JonB can see it. He's done extensive testing with Backup and may have some ideas on what's going on.

Obviously the problem with zip isn't just the typo in that folder name. Since you (I assume) know the problem is stemming from the ulpd section, maybe zipping that separately will do the trick. (That is one of the suggestions for large forums having problems with backing up - zip things by sections or use tar. You're just showing different symptoms - the server isn't throwing a server error or a blank screen.)

Another bit of advice - the Sources, Languages, Templates and html sections don't change much unless you're doing testing, tweaking, or loading mods. That leaves Boards, Members, Messages, Variables, and Uploads as the ones that are the most critical to restoring a damaged forum.

Run two or more batches - the first one with the 'fixed' data can be run once a month or even less often. The second one is the one that needs to be run to safeguard your forum.

(JonB and I have been fighting with Backup for years - seriously, since before 2.5.2 came out - and we haven't come up with any good fixes. )

Edited:

The Bookmark error is in line 732 of Admin/Backup.pm. (Yes, it should be Bookmark.)
See if fixing that fixes the main problem.

There's also an ongoing issue with the zip process throwing errors when a folder is empty or nothing has changed in it.

I have a large forum with lots of attachments...

When I try to backup it using "zip" method, 4 times out of 5, it stops after about 10 MB... Usually it works after I resave the backup settings even if I haven't changed any of the settings. When it finally works, the browser starts downloading the page instead of showing it...

the resulting file reads:

 zip error: Nothing to do! (try: zip -gqr /home/sg120/public_html/cgi-bin/yabb2/Backups/backup.1407103604.src_bo_lan_mem_m
es_temp_var_html_upld.zip . -i /home/sg120/public_html/yabbfiles/BookMarks)
  

I think the error is because it should be "yabbfiles/Bookmarks" instead (lowercase "m").