The first one you posted is about the search index, you may need to rebuild the index. I'm not sure if Lucene search engine supports Greek, so your mileage may vary.
The second post looks like typical things that crawl the web and cause errors, unless you connect an error to an actual user who is experiencing an error I would not be too concerned. I get lots of junk like that in my logs from bots that mangle urls together.
Best,
Joe