Hi Robert, I see this and a similar question that was asked previously.
What preceded this issue? Did you attempt to import a CA signed certificate and then experienced failures?
In the near term, if you're unable to access server manager over either HTTP or HTTPS protocols, we can get you back in by reverting to the default self signed certificate.
1. Stop ArcGIS Server
2. Drill down to \\Program Files\ArcGIS\Server\framework\runtime\tomcat\conf\server.xml. Open this file with a text editor and scroll down to the bottom.
3. Change the value of the keyAlias to keyAlias="SelfSignedCertificate"
like
<Connector SSLEnabled="true" ciphers="TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384, TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384, TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA, TLS_DHE_RSA_WITH_AES_256_GCM_SHA384, TLS_DHE_RSA_WITH_AES_256_CBC_SHA256, TLS_DHE_RSA_WITH_AES_256_CBC_SHA, TLS_RSA_WITH_AES_256_GCM_SHA384, TLS_RSA_WITH_AES_256_CBC_SHA256, TLS_RSA_WITH_AES_256_CBC_SHA, TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256, TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA, TLS_DHE_RSA_WITH_AES_128_GCM_SHA256, TLS_DHE_RSA_WITH_AES_128_CBC_SHA256, TLS_DHE_RSA_WITH_AES_128_CBC_SHA, TLS_RSA_WITH_AES_128_GCM_SHA256, TLS_RSA_WITH_AES_128_CBC_SHA256, TLS_RSA_WITH_AES_128_CBC_SHA" clientAuth="false" connectionTimeout="20000" connectionUploadTimeout="10000000" disableUploadTimeout="false" keyAlias="SelfSignedCertificate" keystoreFile="C:\Program Files\ArcGIS\Server\framework\etc\certificates\arcgis.keystore" keystorePass="not.the.password" maxHttpHeaderSize="65535" maxPostSize="10485760" maxThreads="150" port="6443" protocol="org.apache.coyote.http11.Http11Protocol" scheme="https" secure="true" server=" " sslEnabledProtocols="TLSv1.2,TLSv1.1,TLSv1" sslProtocol="TLS" useServerCipherSuitesOrder="true"/>
4. Start the GIS Server again.
