OK, Ulrike, back to the nitty gritty - lol....
erfurtwiki is what the default wiki in Moodle is based on through Moodle 1.9.x. nWiki is the basis for the wiki in Moodle 2+
The process you describe "should" work, and the fact that you produced a bit of a page suggests that the content of your page caused a problem. The process is a bit tricky because of the play between wiki markup format and html format, and the wiki moves back and forth because the activity provides an html editor.....
If you want, please feel free to upload your text file and we can look at it. It will help a bit for you to become familiar with wiki markup, because that is going to probably be the format least likely to get chewed up in importation. I think the wiki still explains some of the markup if you click on the right little yellow help circle.
You don't need to save it as a .txt file, you just need to save the file as an ascii text file. Text editing on a MS WIndows machine can be confusing because you may not get a pure ascii text file, which causes all kinds of grief. WIndows uses the txt extension for "text" files, but offers a number of different flavors that can be used with that extension.
At some point I documented some tools and techniques one can use to manage such things. Have you looked in docs.moodle.org for additional info on wiki formatting? AHhh, I see a number of the links are broken etc. I did find this discussion: http://moodle.org/mod/forum/discuss.php?d=8501