What version of ikiwiki are you running? I believe this was fixed in [[news/version 3.20140916]], with the patch from [[bugs/garbled non-ascii characters in body in web interface]].
Related reading:
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- [[forum/\"Error: cannot decode string with wide characters\" on Mageia Linux x86-64 Cauldron]]
- [[forum/build error: Cannot decode string with wide characters]]
- [[todo/should use a standard encoding for utf chars in filenames]]