Receiving a "Failed to process template" error message when deploying a new theme pack in Red Hat Single Sign-On (RH SSO)

Issue

We have a new theme pack which we wanted to deploy in RH SSO. However, it doesn't work and the following error message is displayed in the logs:

[0m[31m16:45:51,816 ERROR [org.keycloak.services.error.KeycloakErrorHandler] (default task-1) Uncaught server error: org.keycloak.theme.FreeMarkerException: Failed to process template <TEMPLATE>.ftl
    at org.keycloak.keycloak-services@18.0.3.redhat-00002//org.keycloak.theme.FreeMarkerUtil.processTemplate(FreeMarkerUtil.java:71)
    at org.keycloak.keycloak-services@18.0.3.redhat-00002//org.keycloak.services.resources.admin.AdminConsole.getMainPage(AdminConsole.java:356)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)

...

Caused by: freemarker.template.TemplateNotFoundException: Template not found for name "<TEMPLATE>.ftl".
The name was interpreted by this TemplateLoader: org.keycloak.theme.FreeMarkerUtil$ThemeTemplateLoader@3776b28e.
    at org.freemarker@2.3.31//freemarker.template.Configuration.getTemplate(Configuration.java:2883)
    at org.freemarker@2.3.31//freemarker.template.Configuration.getTemplate(Configuration.java:2703)
    at org.keycloak.keycloak-services@18.0.3.redhat-00002//org.keycloak.theme.FreeMarkerUtil.getTemplate(FreeMarkerUtil.java:85)
    at org.keycloak.keycloak-services@18.0.3.redhat-00002//org.keycloak.theme.FreeMarkerUtil.processTemplate(FreeMarkerUtil.java:58)
    ... 77 more