I have been hired SPECIFICALLY because the senior developer who architecture and coded on his own the main application has no CS background and made such a mess of the code that everything was months behind schedule, and the entire application is locked in an obscure framework.

I was hired despite my lack of knowledge of the used obscure framework, that the senior developer has chosen. However, my knowledge of the language and the fundamentals of the platform are what they needed to fix the bugs and optimize.

I have been doing exactly that for the past 4 months, the number of open bugs at any given time went down from around 70 to less than 6. Application performance and testability have been improving, and overall I think that I was doing objectively a good job.

However in the review the CTO (the person that haired me, and told me about the shortcomings of their senior developer) is telling me that this same person in not happy with the quality of my code.

The senior developer might be scared for his position and trying to undermine me, so that my contract wont be renewed after 6 months?

And even if my contract is up for renewal, do I really want to work in a place like this? Or am I just reading too much into it and I should consider bettering my code?

PS: He apparently didn't give any "constructive" cretinism as in what is bad and what should be made better.