We noticed certain jobs were unable to save config changes, and the attempt to "Apply" config changes lead to a blank error window/pop-up.
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We do not think this is a plugin issue: We attributed this to a plugin-related issue, initially. (We use the Plugin Usage plugin to determine which are in use).

We are seeing the same problem when creating new jobs that do NOT use any plugins.  So, one question is: Regarding the possibility of plugin-related issues: if plugins are installed on the Jenkins server, but the plugins are NOT actually in use for a particular job whose config cannot be saved, then it is possible that one of those plugins is still the root cause of the problem?
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We do not think restarting will resolve the problem: We have restarted the jenkins server, and this did not resolve the issue.
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We do not think its a jenkins version issue: The Jenkins version is Jenkins 2.303.3.
We have another Jenkins env running on a different server, where mostly the same plugins are installed, and this other environment is running the same Jenkins version (2.303.3), but without any such issues.
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We have already checked the logs, currently no errors. We have also checked for possible permissions-related issues with /var/lib/jenkins dirs.
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We do not think it is related to the issue described here: https://issues.jenkins.io/browse/JENKINS-65913 because no deprecated operations are in use for the jobs in question.
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Any help is appreciated - Multiple teams in our org will be affected by this - through the inability to modify existing jobs, or the inability to create new ones. Thanks!