Errors of that sort are usually caused by the entry of information into a blank that is not what the computer expects. Examples: entering a word like "none" when it is asking for a number like "0", putting a dollar sign or commas into a blank that asks for a dollar amount, entering a date in a format the computer can't interpret, entering a long sentence when a short answer is expected, or deleting a "default" entry and failing to replace it with another valid value.
Usually the error message will contain a hint as to which field was problematic--you might read it carefully to see if you can discern the problem. For example, it might say "May 1, 2011 is an invalid date or time string" which is a hint to look for the place where you entered "May 1 2011" and replace it with "5/1/2011". Or it may say "No must be interpretable as a valid number in the current locale" which is a hint to look for the place where you may have entered "No" in a blank that was anticipating a number.
Clicking the "back one page" button on your browser (the left-pointing arrow) will return you to the data entry page so that you can try again.
If you experience this problem repeatedly, please cut and paste the error message into a ticket and I will be happy to help you sort through it.