First of all, I note that you have a SubVersion (SVN) message there. Since you are using a single-file .hmxz project, you cannot meaningfully use SVN. The .hmxz file is a compressed binary (actually a Zip) that SVN cannot do anything with. You need to use the uncompressed HMXP format, so that SVN can version all the individual project files.
Before doing anything else, please first make sure you are using the latest version of Help+Manual 6, which is 6.5.6 Build 3027. Check with Help > About and download the installer here
to update if yours is older.
Then move the project files you are trying to convert out of your SVN system (you also cannot manage HM3 projects at all meaningfully with SVN, since they only use compressed binaries) and try the conversion again. If you continue to have problems, please send the HM3 projects to support AT ec-software.com
(replace the AT with @) and we will check them for you.