[OJS 3.3.0.7] Crossref Credentials not saved into the plugin and the plugin does not display citations

[devlinbiezus]

Describe the bug
In the plugin settings, when we enter the Crossref API username and password and Clive saves the changes, they are not reflected in the plugin. Additionally, the citations are not being displayed on the article landing page.

To Reproduce
Steps to reproduce the behavior:

1. Go to Settings
2. Enter the Crossref credentials
3. Click Save
4. Open the Settings again, the credentials are blank

What application are you using?
OJS 3.3.0.7

Additional information
Please add any screenshots, logs or other information we can use to investigate this bug report.