xWiki LTS Upgrade 12.10.10 -> 13.10.3 HTTP 500 Error and NullPointerException

We are running xWiki on LTS-version 12.10.10. The following updates ara available:

xwiki-common/lts 13.10.3 all [upgradable from: 12.10.10]
xwiki-mysql-common/lts 13.10.3 all [upgradable from: 12.10.10]
xwiki-tomcat8-common/lts 13.10.3 all [upgradable from: 12.10.10]
xwiki-tomcat8-mysql/lts 13.10.3 all [upgradable from: 12.10.10]

Installing those updates works just fine. But when I try accessing the wiki, I get a HTTP 500 Error and a NullPointerException from java.

HTTP Status 500 – Internal Server Error

Type Exception Report

Description The server encountered an unexpected condition that prevented it from fulfilling the request.



Note The full stack trace of the root cause is available in the server logs.
Apache Tomcat/8.5.39 (Ubuntu)

I already had the same problem when trying to update to 13.10.2, but I waited until 13.10.3 to ask here.
Do any of you have an idea why this is happening?

This error does not really give any information (it’s just a consequence of a not fully initialized XWiki). You should take a look at the very first error in the application server log.

Okay, thank you for this info. This helped me solve the issue.
I checked the log before, but now I found a message that /var/lib/xwiki/data/store/solr could not be deleted.
And because I also have problems with SOLR (to be discussed in another forum post), I went ahead and deleted that folder manually.
This made the error go away and I am now able to start the wiki again.