I am facing exactly the same problem.
I have developed a relatively complex form using Adobe LC Designer 8.2 and I applied Reader Rights from LC RES. Also, I used Digital Signature with the feature to lock fields in a given collection after signing.
Surprisingly, I am getting alert "<field name> validate failed." for the first 4 fields in the form, in the following condition:
1. When running the form from Adobe Reader 8.1.x.
2. After click the Signature Field to sign and save the form.
On Acrobat 9 Pro, it works fine.
Note: There is no validation script nor anything that will trigger this validate event.
I am having the same problem. When a digital signature is applied, I get an error pop-up for all of the Text Fields that are set as "required." It will cycle through each one successively two times within each "Group." It does this even when the required fields are completed. I don't have the error boxes checked for scripting or validation pattern. I get the error pop-ups in the same fashion when I try to close the document as well. Note that this only happens with text fields and not date, state or numeric fields. I am about to give up and just tell users to ignore the error messages!
I found that my issue was related to not setting the fields correcting in the Object > Field > Signature tabs:
I found that my issue was related to not setting the fields correcting in the Object > Field > Signature tabs, see figure above. Once I set the fields correctly to be locked by the signature, it worked fine.
Thanks for your question and I hope this helps.