I am getting an error while trying to add a new item in the custom list..it was working before, but now it is throwing the following error
As chris is saying, please look in to your ULS logs. This will say where the problem is occuring.
How to do so? Do the same operations again, so the error occures again. Copy the correlation ID (without leading and tracing spaces) and open the latest log file (found in 14\Logs) and search this document for ur correlation ID (control + f).
Check everyline that is showing "Unexpected" AND includes your correlation ID. If you have no idea what to do, feel free to upload a link to your log file and give me your correlation ID. I can have a look for u.
This error message occurs on SP 2007 because of workflow, sometimes. You can deatach it (thru list settings) and attach it again (thru designer, open and save), if it helps.
And second thing, make sure, that you don't copy text to multiple text fields from external programs, field preserves formatting and imported html can be wrong without end tags.