I have the current version of Serendipity having just installed it.
I discovered as an administrator I could add users. I added two users with passwords and standard access rights.
When I went to add a "third" (actually 4th if you count the blogadmin user) the screen load can not complete the load. I think all but the "save" button is being displayed.
Is the MYSQL database corrupted? I'm too new with this product to suggest what else it might be
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used phpMyAdmin to access the databe. I added a user there. When I came back to the blog it had the new user and it now allowed me to add more users using the blogs Manage User feature.