I have no way of knowing why you suddenly get that error.
In addition to disable the search index the only suggestion I have is also delete all rows from mp_IndexingQueue and mp_TaskQueue then recycle the app to clear the cache.
That is a very outdated version of mojoportal (released in Nov 2009 so 6 years old) that I would not have in production for any of my customers. There have been important security updates since that version so I would never use it for any site at this point. But it is also so old that upgrading will likely have some challenges. ie upgrading would likely need a change in hosting to a newer version of .NET framework since the latest mojoportal requires .NET 4.5. Likely to have some skin issues and javascript issues to work through after upgrading.
Nevertheless, in spite of the challenges upgrading is what is called for if the site is important to anyone.