there is a bug in the last xinha release (Xinha 0.96 Phoenix beta2) included in OpenACS that causes entries of the xinha toolbar (also defined via HTML-form elements) to be transmitted to the backend (which does not know about these). xowiki warns you about this.
You have at least two options:
a) disable the xinha plugin 'LangMarks'. If you have not configured it for the richtext entry, you might have it configured via the package parameter XinhaDefaultPlugins in acs-templating.
b) update to a newer version of xinha (get the nightly tar ball from http://trac.xinha.org/wiki/DownloadsPage and unpack it over acs-templating/www/resources/ .... after having made a backup).
In general, i would recommend to remove all plugins from XinhaDefaultPlugins where you don't know what they are doing or that you don't need. Most probably, it would be a good idea to set the list of default plugins for OpenACS minimal (maybe empty).
Let me guess why you are now posting this message and you have not seen this behavior before: previously, you have used in your "old installation" and some older version of xinha, not having the problem. In the new installation, that i have recommended, you see now the problem. it would really help, if you include such context information in your posting to reduce my guesswork. of course, i can be completely wrong on this.