How to resolve database version mismatch? / Database version [10.0.11] Does not match application version [10.0.21]


We are using Promise CE update 3 -

One user gets the following prompt on startup, and cannot use Promise

I have several questions,

  1. What has caused the mismatch version, if we all have the same application version?
  2. How do I check which version database is on the server? (I know from the above image it's 10.0.11, but I'd like to know how to check in the future)
  3. How do I know which application version uses which database version? 
  4. If that user upgrades the database, how will it effect the other users?



Possible answer to question 3, found a table of Application and Database versions here Project Database Version - Substation  But not sure if it is applicable to Promise as well as Substation. The answer to 2 is also on that page.