I set the OJS theme to a new theme (themeX) in the ojs/manager/setup/5 and also in the ojs/admin/settings.
It was working fine.
Then I was trying to update the theme, for that, I deleted the themeX folder from the ojs/plugins/themes folder, and instead created a fresh new folder under the same name (themeX), and put the updated theme files there.
After that, OJS is not working properly, i…e I cannot open pages: ojs/manager/setup/5 and page ojs/admin/settings to modify the look of the journal anymore. On accesing these two pages, nothing shows at all, just a blank page. Everything else (as far as I can tell) is working properly. I can even access other setup pages, such as ojs/manager/setup/4.
I tried clearing the data cache and the theme cache from the admin menue. but it did not help.
I have a backup of the server (not the OJS). How can I correct this issue? Should I restore the OJS files? Which folder? Or should I restore the OJS database? how?
OJS version 22.214.171.124
 I also noticed that the page ojs/manager/plugins is also not accessible.
Can it be that the themeX folder was also in the cache folder and hence causing the error?
if yes, how can I clean cache in the future?
I tried deleting the cache folder, but it crashed OJS completely.
I tried CLEAR DATA and CLEAR CACHE options from within the ojs, but it did not help in the case of this issue.
I’m not aware of any native functionality that would create a copy of the class in the cache directory.
To find the offending file(s) if this were to happen again, use a search tool such as grep to look for the duplicate class definitions: grep -r 'class themeXThemePlugin extends ThemePlugin' plugins/themes/
Using the options in OJS to clear the cache is appropriate. If you were to run across a situation where you needed to clear the cache without access to the UI, you could remove the files from the cache directory and subdirectories, leaving the directory structure intact.