Hi Bill,
If its a multi site installation hosted in Medium Trust it could be the problem, or if you have this setting in Web.config/user.config with an invalid path or a path without write permission:
<add key="Lucene.Net.lockdir" value="D:\__projects\mojoportal\joe\Web\Data\Sites\1\index" />
note the value is just an example, it would need to be a real server path on your server and this setting would need to be correct in Medium Trust but it could be a problem in multi site installation since it would use the same lock directory for all sites.
If that is the case, it should be fixed in the next release of mojoportal so that this config setting is not needed and it uses the site specific index folder automatically as the lock directory.
If running in full trust, this setting is not needed in the current release, but if it exists and has an incorrect path it could also cause the problem.
Hope it helps,
Joe