It seems add-on installation was initiated on 2017-12-20 but is currently stuck, which is preventing other plug-ins from being installed until current task is complete. Usually, add-ons don't take more than few minutes to install, so it's not normal for it to be in this state. Best way to know where it's stuck is to go through catalina.out log to check where it might be stuck or if there are any errors, especially associated with a date stamp in question.
You can try cleaning your plugin cache by following instructions from How to clear Confluence plugins cache KB:
It might help clear whatever is prevent installation from completing and allow you to install plugins again.
If you find any relevant errors in catalina.out log or if plugin cleanup does not solve your problem, please let us know.
Two vulnerabilities have been published for Confluence Server and Data Center recently: March 20, 2019 CVE-2019-3395 / CVE-2019-3396 April 17, 2019 CVE-2019-3398 The goal of this article is...
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