KB article: https://support.atlassian.com/jira/kb/temporarily-changing-the-logging-level-doesnt-work-and-no-error-is-shown/

Quote from the article:

Log4J has been updated to version 2.x on Jira 9.5.x and new JVM arguments have been included by default in the setenv.sh and setenv.bat files coming from fresh Jira installations:

-Dlog4j2.contextSelector=org.apache.logging.log4j.core.selector.BasicContextSelector -Dlog4j2.disableJmx=true -Dlog4j2.garbagefree.threadContextMap=true -Dlog4j2.isWebapp=false

Jira admins performing system updates or upgrades might rollover changes included by default in these files if they

[...]

To solve the problem you need to add back the 4 JVM parameters listed in the Cause section on the line starting with "JVM_REQUIRED_ARGS", the full line can be seen below:

JVM_REQUIRED_ARGS='-Dlog4j2.contextSelector=org.apache.logging.log4j.core.selector.BasicContextSelector -Dlog4j2.disableJmx=true -Dlog4j2.garbagefree.threadContextMap=true -Dlog4j2.isWebapp=false -Djava.awt.headless=true -Datlassian.standalone=JIRA -Dorg.apache.jasper.runtime.BodyContentImpl.LIMIT_BUFFER=true -Dmail.mime.decodeparameters=true -Dorg.dom4j.factory=com.atlassian.core.xml.InterningDocumentFactory'