When opening various dialog boxes in the Rich Text Editor (for example, Hyperlink Manager), the following error might occur in the opened dialog box if the Sitecore application was restarted after the editor had been opened:
Cannot deserialize dialog parameters. Please refresh the editor page.
Error Message: The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters.
Configure a static machine key for your solution, by adding an entry to the <system.web> element in the web.config file.
For example:
<machineKey
validationKey="C22F04044FD625D2027C8B3AEF69C8D9A1046572290F8C2699AD6FF31B178548D2C030504CD7E1A171A4D9A171F23228163B90DDEB6E9AF2B9D5211DC96B89A2"
decryptionKey="AA70A2D77469C491E426E6C92FD4C05D8785B1E0673EF6B311BA357DAC06D5F9"
validation="SHA1" decryption="AES" />
Note that the sample above is an example only. Do not use these values in your solution, but generate them using external tools like the following:
https://learn.microsoft.com/en-us/archive/blogs/amb/easiest-way-to-generate-machinekey
Reload the Sitecore Client and try again. You can reload the Sitecore client by pressing Refresh in the browser or you can log out of Sitecore and log in again.