You will see this message when you enter a wrong password or use the wrong keyfile/don't use your keyfile. If you change your encryption algorithm in settings after encrypting files, then you will also see this message when trying to decrypt files.
If you are entering the correct password, using the right keyfile (if applicable), and are using the correct encryption algorithm, then that means the file has been modified and some data required for decryption may be missing.
If you see this error when trying to encrypt files or use the 'Password Sharing' feature on Windows, then you should install the Visual C++ Redistributable for Visual Studio 2015-2019 to fix the problem.
This error may also indicate a bug in the program, so please report the error as outlined here.
These exceptions will occur when you select files to encrypt/decrypt that are inaccessible to Kryptor because Kryptor does not run with administrative privileges. You can move inaccessible files to an accessible location (e.g. your Documents) to encrypt/decrypt them.