For one user, one Clarity project or program won't open in Microsoft Project (MSP) intermittently or it takes a long time to open. It hangs on the opening XML step. The below error message is then seen when trying to open the project after the issue occurs:
"The last time you opened <project> it caused a serious error. Do you want to open it again"?
Reference: MSP Error: "The last time you opened 'ProjectID' it caused a serious error..." when attempting to open a project from Clarity
If the above doesn't solve your issue, see: MSP opening projects issues