JS error when commenting on an issue

Steps to reproduce:
1) retrieve an issue
2) attach a file
3) from the same screen (don't click on view issue link), press on link "Comment"

Before displaying the add comment form, an error pops up, either:
Line: 390
Char: 1
Error: Object expected
Code:0
URL: http://.../secure/ManageAttachments.jspa?id=10248
or
Line: 410
Char: 1
Error: Object expected
Code:0
URL: http://.../secure/ManageAttachments.jspa?id=10451