strange problem - not sure what causes it

I do have a simple custom content type with a few fields, including date and text choices.

a) it works all fine using it as administrator, create, view, edit, pre-view edit, submit...
b) I be leave I set all access permissions correct for normal users
c) as normal user I can create and view and also open the edit form -- ok
d) but: if I as normal user go to edit and the preview, I get the warning messages below, same for submit/save -- i.e. user can not save any changes.
e) also strange: once save/pre-view hit the edit-form shows a option-select field with red frame and no option to selecty any longer.
f) this is reproducable for any post of this type.

# warning: date_format() expects parameter 1 to be DateTime, null given in /mnt/stor2-wc2-dfw1/431807/www.sbraweb.org/web/content/modules/date/date/date.module on line 356.
# warning: date_timezone_set() expects parameter 1 to be DateTime, null given in /mnt/stor2-wc2-dfw1/431807/www.sbraweb.org/web/content/modules/date/date/date.module on line 358.
# warning: date_format() expects parameter 1 to be DateTime, null given in /mnt/stor2-wc2-dfw1/431807/www.sbraweb.org/web/content/modules/date/date/date.module on line 360.
# warning: date_offset_get() expects parameter 1 to be DateTime, null given in /mnt/stor2-wc2-dfw1/431807/www.sbraweb.org/web/content/modules/date/date/date.module on line 362.
# An illegal choice has been detected. Please contact the site administrator.
# An illegal choice has been detected. Please contact the site administrator.
# An illegal choice has been detected. Please contact the site administrator.
# An illegal choice has been detected. Please contact the site administrator.

see also attachement screenshot and http://sbraweb.org/?q=node/2087

Any idea how to start tracking down the issue?
Thanks a lot.