You wish to change the look of the default error pages and refer to SAP official documentation which suggest set ErrorPageFragments.properties file in folder \usr\sap\<sid>\<instanceid>\j2ee\cluster\bin\services\http\lib\private but the file appears to be missing.
SAP Netweaver AS JAVA 71X/72X/73X/74X
KBA , BC-JAS-WEB , Web Container, HTTP, JavaMail, Servlets , Problem
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