I had this problem (discussed in this thread: https://groups.google.com/forum/#!topic/TiddlyWiki/T6WkSLAzjVc). If it's the same thing, then it's due to a bug in the prerelease. I solved it by using a different upgrade path. If you do a filter search for [all[tiddlers]] then export the results to a JSON tiddler file, you can then drag the resulting file onto an empty v5.1.8 tiddlywiki file from tiddlywiki.com, and you should be back to a bug-free version.
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