Major reasons why you get an error while signing forms are as follows:
The size of files e.g. Memorandum and Articles of Association attached by you is large. File size for each of these documents should not exceed 2 MB. You can reduce file size by keeping low resolution while scanning documents.
Another reason might be that you are trying to sign forms for all persons from one login. You have to login through each User ID to sign forms.
Compatibility view is not enabled in Internet Explorer. Please add SECP website to Compatibility View under Tools> Compatibility View Settings.
Security Settings for java are not set as per requirement.