I don't have 'the' answer for you based on the question. Here are some things I would check or try. These things are not that uncommon and it's a bit fiddly to fix.
Auto Calc - File > Options > Schedule - Usually this is checked. There are good reasons to uncheck / turn it off of course. May seem obvious but I overlook it all the time.
Scheduling Messages - File > Options > Schedule - Often these get turned off, turning them back on could reveal a message that is relevant to the problem.
Advice from Planning Wizard - File > Options > Advanced - same train of thought as for the Scheduling messages
.NET Framework - If this is not current MS Project gets cranky. The latest .NEt is 4.6 I assume MS Project itself is patched up current too.
I have seen issues if say MS Project was installed and then Office or an Office program was installed after. I try to install & update MS Project after Office and then update .NET
Sometimes the task just gets corrupt and it has to be recreated, and re-linked. I will typically create a new task, copy/paste the critical info out of the old one, like predecessors & successors, and actual dates, then delete the old one. Copying the entire task usually just moves the problem to the new task. Yes, I have seen these things happen for one user and not the other.
If it starts cropping up on other tasks or other machines, then I usually export the schedule to .XML, close project, re-open and then open the .XML file as a new project. This has the effect of purging the data base of anything Project doesn't like for lack of a more technical explanation. Oh, then save this new project as an .mpp with the appropriate name, of course.
Hope that helps.
-j