@Ganzjahresgriller is right. The best we can do is throw in some wild guesses. I followed this guide years ago and have updated Roundcube multiple times since, always following the upgrade instructions from the same guide, and it still works. So chances are, you may have missed a step or made a small mistake somewhere, maybe a typo.
That being said, ‘Internal Error’ usually indicates that the front end cannot connect to a necessary back-end service. In this case, it is probably failing to connect to the database or postfix/dovecot, which can happen for a number of reasons. For example, if you made a small mistake here, or here.
Even something as small as a missing ; or an extra one in a PHP config file can invalidate the whole file. When that happens, the frontend doesn’t know what to do, and throws an Internal Error.
