ISPConfig does not damagae group file which has the correct format. The group file must have been corrupted before you edited this user or another application was editing the group file at the same time. Do you run webmin on the server too?
To let ISPConfig recreate the groups, execute the following SQL staement in the ISPConfig database:
update isp_isp_user SET status = 'u' where status = '';
Then edit a user in the interface, change something and click on save.