See Cindy10's thread Norton Blocked my Mailwarebites antimailwareprogram! in the Norton forum for further information about these Unauthorized Access Blocked messages. As others have suggested, these entries are logged in the Norton Product Tamper Protection (NPTP) section of your Norton security history every time any executable (including trusted Windows system files like svchost.exe, dfrgntfs.exe, etc.) attempts to read/write/edit/delete a Norton file. This NPTP logging rarely causes a performance issue and it's usually safe to ignore these Unauthorized Access Blocked entries in your security history.
The instructions in Cindy10's Norton forum thread are a bit out-of-date but if you'd like to try Firefox's suggestion in post # 3 to create scan exclusions for the Malwarebytes v3.x files listed at https://support.malwarebytes.com/docs/DOC-1123 (like Firefox, I always do this to prevent possible conflicts) this will likely stop the Unauthorized Access Blocked logging for MBAMService.exe. Go to Settings | Antivirus | Scans and Risks | Exclusions / Low Risks and then add the file and folder exclusions in both the Items to Exclude from Scans section (to prevent detection during a "traditional" file scan like a Quick Scan) and Items to Exclude from Auto-Protect, SONAR Detection and Download Intelligence Detection section (to prevent detection by Norton's real-time protection) as shown in the image below. See the support article Exclude Files and Folders from Norton Auto-Protect, SONAR, and Download Intelligence Scans for further information.
I'm currently having a few issues running Norton v22.x and MB Premium v3.x together in real-time protection mode on my old Vista SP2 machine so I'm also testing the following exclusions for Norton executables in Malwarebytes at Settings | Exclusions | Add Exclusion. Unfortunately, the location of the main Norton executable (nortonsecurity.exe if you are using the latest Norton v188.8.131.52) moves to a different folder every time Symantec releases a product update so if you want to create an exclusion for a specific file like nortonsecurity.exe you'll have to remember to edit the exclusion every time Norton is updated.
---------- 32-bit Vista Home Premium SP2 * Firefox ESR v52.8.0 * Norton Security Premium v184.108.40.206 * MB Premium v220.127.116.112-1.0.365