I've been having the same problem for about 10 days now. When I weigh-in, it actually does update my weight that's shown in the top right-hand corner of my page, but it won't update my weight within my Challenges.
I also get the same error when I change what shows in the columns (calories, fat, net carbs, etc.) where I enter my food eaten for the day. I change what I want to be shown, click "Save", and I get that error page. I have noticed, though, that when I go back to the "Enter Food" page, it has made my changes.
I've tried with Google Chrome and IE, and also tried on a different computer with a different OS, but still get the same error page in both "weigh-in" and changing what shows in the food diary columns.
I also contacted FatSecret and got an email response stating:
"Many thanks for contacting us and sorry for the inconvenience.
We're still not sure why you are experiencing these errors, please bear with us in looking into this further.
We appreciate your patience and wish you well.
The FatSecret team"
"This one step - choosing a goal and sticking to it - changes everything." - Scott Reed