Index file not readable

I've installed SLA on my site, and for several weeks all went fine. Then suddenly the search stopped working, throwing the following error: "File 'sites/default/files/luceneapi_node/segments_226' is not readable. Error thrown in .../sites/all/modules/luceneapi/luceneapi.admin.inc on line 430.". I had a look at it, the file segments_226 doesn't exist, instead there's a file segments_229.

Perhaps I'll have to rebuild the complete search index, but how can I accomplish this? Delete the whole luceneapi_node directory? Or just some files? The builtin index optimization doesn't work anymore, index statistics throws a file error, too.

What should I do?

Regards,
Boris