Google Product Forums
Re: Why do I get "Server rejected" messages when I try to upload my pictures?
inexplicable_hitec Apr 18, 2011 11:24 AM
Posted in group: Picasa

Categories: Picasa Web Albums :

I had a file named "2011-03-20 13.27.12.jpg" that used to give me this error. I renamed it to "2011_03_20 13_27_12.jpg" and it uploaded alright.

Hence,

2011-03-20 13.27.12.jpg       - failed
2011_03_20 13_27_12.jpg    - worked

I guess a period or a hyphen in the filename is a problem. YMMV