Let's wait for the developer to have a detailed answer. But did you successfully installed the latest version using the manual upgrade instructions show here? -> http://b2evolution.net/man/installation-upgrade/upgrading/upgrade-instructions

If you are using Cpanel, did you try to run a repair on your database?

@ecmedia it looks like you have deleted the install folder as instructed in the Step 7 of the upgrade process, this has nothing to do with the error itself, but makes impossible to run the upgrade routine again. Did you make a backup before to start the upgrade? Could you restore your site back to 4.1.5?

The error you reported in point 4 is related with one of the automatic steps performed by the upgrade script, this is not supposed to occur in healthy databases, but try to focus in restore your site back and then, lets try to solve that issue.

Regards.

1. Yes, I deleted the install folder as instructed because I did not want my website to be compromised (while I try to find the answer here). I can copy the folder again to the server if I want to run the install script again.

2. Apparently, I could not make a backup using the cpanel softaculous script because it always got stuck at 95%.

3. Yes, I tried to repair the database (when I read on this forum that database can be damaged when upgrades are forced and that happened in the past) using the cpanel repair feature but I keep getting the same error messages. Meaning, DB cannot be repaired.

4. I don't have the backup that one gets after using the script, but because I was desperate and lack tech expertise, what I did do was to use a FTP software to download the whole blogs folder to my laptop. As a last resort, if I upload these files back to the server, do you think that will restore it back to 4.1.5.?

1. Yes, I deleted the install folder as instructed because I did not want my website to be compromised (while I try to find the answer here). I can copy the folder again to the server if I want to run the install script again.

Please restore the install files and run the upgrade once again.

2. Apparently, I could not make a backup using the cpanel softaculous script because it always got stuck at 95%.

Do you have access to the phpMyAdmin through the cPanel? Making the backup with this tool could be easier.

4. I don't have the backup that one gets after using the script, but because I was desperate and lack tech expertise, what I did do was to use a FTP software to download the whole blogs folder to my laptop. As a last resort, if I upload these files back to the server, do you think that will restore it back to 4.1.5.?

Yes, please get rid of the 5.0.7 files and try to put the 4.1.5 files back to your site, that should let you to be online again while we try to help you.

Thanks for the tips.

1. I followed your suggestion to try an upgrade once again but same errors.

2. Regarding the backup, I learned that my hosting company automatically backs up the whole website, so I did find an older version, and I was able to restore that to the version I had before, that is 4.1.5 and the website http://mynippon.com/blogs/ is showing that version but it looks like the database schema is still an issue. Right now, it would be great if at least I can restore the website to show up again because it did bring some revenue (it has a few hundred posts that bring traffic).

3. I also found a backup of the blog that is in the format softsql.sql on my server because softaculous script saved it. I just don't know what to do with this massive file.