I have seen this before with various Bitcoin forks.
Some wallets expect a string while others a float. For this reason, the plugin first tries to withdraw first by specifying the amount as a float. If the result code is 500 the plugin retries the withdrawal by issuing the amount as a string. See here:
For some reason this did not work with your MMC wallet.
Please let me know which coin this is (there are several with this ticker symbol). I can then try to reproduce the issue with that wallet and see what the problem is. If you have a github link to the wallet please also provide this, and the commitish or tag or wallet version that you are using.