Thanks and sorry for the delay, I had to struggle with severe problems in other projects.
Could you tell me what fields you were you trying to import with those characters? Were those in the username, or other fields?
Yes, I know and kept username and similar without special characters. These were all custom-fields.
When you create the user from WP Admin > Users > Add New , can you enter those values without issue?
Yes, they work fine.
In the meantime I came up with a detail that I might have overlooked myself.
German versions of MS Excel and Open Office Calc are usually set to “ISO 8859” by default, not to “UTF-8”, so I probably saved the data in this format.
Is it possible that the CSV import/export in s2Member only works properly with “UTF-8”?
@ Cristián: If the CSV import/export generally works with special characters I would test it again.
The reason why I ask is: I had to deal with wordpress-plugins that could not handle special characters correctly at all and that could not be solved (wasted a lot of time on this).
Nevertheless, I assume, that s2member is programmed more accurate overall. If you say it should work, I believe you.
Best regards and many thanks!