I set up a brand new Confluence 5.1.4 site, loaded it with only the demonstration site, and then immediately performed an XML backup (attached).

I then set up a brand new, empty Confluence 5.2-beta1 site and set it up with a production database (using the "Empty Site" option, with no additional content created).

On 5.2-beta1, I then went to Admin->Backup & Restore->Restore, and told it to restore the 5.1.4 backup (using the default "Build Index" option enabled, in case it matters).

The restore seemed to work OK (see screenshot) and I was able to navigate the restored site OK.

However, after shutting down Confluence, it refused to start up again with the following error:

2013-07-24 15:31:25,962 ERROR [main] [atlassian.confluence.setup.BootstrapApplicationStartupListener] checkConfigurationOnStartup Confluence will not start up because the build number in the home directory [4381] doesn't match the build number in the database [4249].

The attached atlassian-confluence.log parallels this process:

• The initial 5.2-beta1 installation (successful) was performed at 07-24 15:24:07.
• I started the import of the 5.1.4 data at 07-24 15:30:12.
• I tried to restart Confluence again at 07-24 15:31:12 (failure).

This is obviously a simplified example, but I also saw similar problems when trying to import a 5.1 XML export to 5.2. Other upgrade procedures seem to work better: at least with the 5.1 import, if I shut down Confluence, wiped out the SQL database, and then redid the import from the setup wizard, the build number would "stick".