`text/xml;charset=UTF-8` error (3.4.0-3)

I am using OJS for about two years and I must say I am just an editor and not programmer so the problem is quite baffling for me.

I recently connected my journal with WorldCat. Unfortunately, I decided to upgrade the already issued articles sitting in the repository in line with the new formatting I applied to the Dec 23 issue. With the deletion of the old articles and replacing them with the new ones, I got email from WorldCat Digital Collection Gateway that the repository was not matching with the already given base url in the DCG. I approached them, they tried their best and the last email I got from them was this error in the subject text/xml;charset=UTF-8

Please let me know how to remove this error. As data is not being accepted in Open OAI too. I am very much concerned because I want my journal to reach bigger audience.

What application are you using?
I am using the OJS version before this new update

Please tell me if I upgrade to the newest version, will this problem be solved or I will have to install ojs again and then shift the data to new sight?

I experienced the same thing, how do I solve this problem?