Angels have the Phone Box
Hi and welcome to Tenforums.
MBAM is an anti-malware program, while Defender is an anti-virus. MBAM works alongside AVs in harmony on a system - even the Pro (paid) version which is always active (as opposed to the free/passive version).
Personally, (and this is just my personal opinion), I would suspect that Defender is having difficulty with something lurking on your system.
One thing to do, would be to download and install a 3rd-party AV (like Avast Free, or the 30-day trial of ESET NOD), which will auto-disable Defender as it takes over. Once it is installed and fully updated, I would run a complete scan, including scheduling a boot-time scan (which can be found in the settings in Avast.)
If you have any problems installing Avast/ESET (or whatever program you choose), it could be a sign of something bigger.
You could also run the System File Check to make sure everything is intact.
SFC Command - Run in Windows 10 - Windows 10 Forums