First I suggest upgrading to version 6.3.9, take a look at next documentation section https://docs.openkm.com/kcenter/view/ok ... guide.html
About the preview error you should share the catalina.log file to check the issue:
1- stop openkm
2- delete all files into the logs folder ( the logs folder is into tomcat home folder )
3- start openkm
4- preview a document what it is raising the error
5- share the catalina.log file in this post
About the preview error you should share the catalina.log file to check the issue:
1- stop openkm
2- delete all files into the logs folder ( the logs folder is into tomcat home folder )
3- start openkm
4- preview a document what it is raising the error
5- share the catalina.log file in this post