% is a special symbol for fields in the file name.
If you paste your exact “Archive Name” field, we can perhaps see what you need to change.
I am using this:
backwpup_SOMETHING_%Y-%m-%d_%H-%i-%s
Preview: backwpup_SOMETHING_2014-11-13_00-47-40.tar.gz
No spaces, case sensitive, and if you mouse hover over the field the full list of what you can use should display.
Replacement patterns:
%d = Two digit day of the month, with leading zeros
%j = Day of the month, without leading zeros
%m = Day of the month, with leading zeros
%n = Representation of the month (without leading zeros)
%Y = Four digit representation for the year
%y = Two digit representation of the year
%a = Lowercase ante meridiem (am) and post meridiem (pm)
%A = Uppercase ante meridiem (AM) and post meridiem (PM)
%B = Swatch Internet Time
%g = Hour in 12-hour format, without leading zeros
%G = Hour in 24-hour format, without leading zeros
%h = Hour in 12-hour format, with leading zeros
%H = Hour in 24-hour format, with leading zeros
%i = Two digit representation of the minute
%s = Two digit representation of the second