Crash on trying to delete / modify Custom Field and no way to remove field

CiviCRM in Joomla 1.7.2
Created a new set of custom data
Created a new field with multi-select
Then I wanted to change the field type but when I saw the warning about beta state preferred to delete the field and create a new one.
Deleting the field the aplication crashed with following error description:
"Sorry. A non-recoverable error has occurred. DB Error: not found Error Details Return to home page."
Entering another time I found that all select items are removed and there is no way to create new ones nore to change the field type nor to delete the field. The only way to use my custom data is to deactivate the field.
(By the way, when a item is deactivatet the style - struck out - includes the text of the bottoms - not really nice)