'Add a new tag' in item edit doesn't allow old tags addition

I have searched through the forum, but I can't find a previous tread reporting that (may be my fault...): editing an item and trying to add a tag, the tag list doesn't appear anymore when trying to add a tag that is already present in the tags table. As result, it's impossible to add that tag to the item (only new tags are allowed).
Tested on Firefox 11.0/IE 9, Joomla 1.5.25, FLEXIContent 1.5.6RC4 r1209

hello did you update joomla and flexicontent ?

Yes, I did it: currently running on Joomla 1.5.26 & Flexicontent 1.5.6 RC4 (r1235), IE 9 & FF 11.0. More exactly, it starts to happen since you have added a few tags (I don't know after which number...): when tags list is empty or short, it works normally; but since a few tags are added, it doesn't work anymore. If you start typing a tag already in the list, the animated pic starts to run, but nothing happen, and you can't add that tag to the item. You can however add a new tag.