かねる

Unable to do
ます stem + かねる

(兼ねる) かねる has many meanings, but the one we’re going to discuss here is its usage as a suffix.
As a suffix, it means “Unable”.
This suffix is mostly only used in Keigo, to express a negative without saying no or using ない or ません.
かねる is rarely used with potential forms.

Examples

• 申し訳ございませんが、お答えいたしかねます – Terribly sorry, but we are unable to answer your question.
• 提案に賛成足しかねます – Unable to agree with the proposal
• どこで働くのかは決めかねます – I am unable to decide where I want to work.
• 無料でご案内しかねます – Unable to guide you free of charge. 
  
  

かねない

Possibly/Likely (Bad thing)
ます stem + かねない

かねない means “Not unable to do” or more simply “Possibly”, and sometimes “Likely”.
It is only used for bad events.

It is most similar to おそれがある (There is a fear that) (Source)
It is also similar to がち, but is more about possibility than tendency. (Source)

かねない is almost never used with potential forms. 

Examples

• 事故が今でも起こりかねない – An accident could happen even now.
• やりかねないミス – A mistake that could happen.
• コロナの影響で色んな会社が倒産しかねます – As a result of Corona, a lot of businesses could go bankrupt. 
• 彼女ならやりかねないことだ – Its something (bad) she might do.
• 鍵のチェックは忘れかねません – Its possible/likely to forget to check the keys.