i have had the same problem. at my system this was NOT a problem with suexec.
The problem was that ISPConfig wrote phpadmin_xxx properties inside the OLD vhosts.
If you change to suphp that this old entries made the problem.
If there is a problem starting the apache, ISPConfig will use the OLD config instead of the new one.
to avoid the problem:
1) make a backup of the old config
2) edit the config and remove all the phpadmin entries
3) try to restart apache with the "new" file
4) enable suphp
5) let ISPConfig write the "new ISPConfig" file
i hope it is clear what i mean. if not ask ad i will help you more!