Code that worked for years suddenly glitches - CF2021 > CF2023

I've read all the replies so far. If you remain stumped after doing what bkbk and Pete have suggested, then please do this:

1. Dump cgi.SSL_CLIENT_S_DN_CN right before the code that's failing. Is it indeed an array? And is array element 1 what you'd expect?
2. Dump to_char(dateObject, 'YYYY-MM-DD') and separately just dateobject just before the line using those. Do those have what you expect. 
3. Please don't presume other similar dumps in other places prove "you've done it already": do those, right there (to a file if you must). Let us see the value of the date dumps. 
4. If you remain stumped, you may be presuming there's a problem with CF or perhaps your application.cfc/cfm. In that case, please create a standalone few-line program that demonstrates the problem.
5. Then try running that on its own folder with its own empty application.cfc. Does the error/unexpected behavior persist?
6. If so, run that code on cffiddle.org or trycf.com, both of which will let you run it against different cf versions.
7. If it fails on them, on different versions, do you have some environment where this demo code runs well? If so, tell us it cf version and update level that is. Maybe it's on an older update (those sites don't let you try different update levels). You might want to share the few-line demo here for us to try. Some of us can test on different update levels. 

• "Position 1 of the array doesn't exist"
  My guess is that sometimes cgi.SSL_CLIENT_S_DN_CN does not contain a ten-digit substring. You can verify this by using try/catch.
  In the following example, when the error occurs, the CGI scope is dumped - out of the user's sight - as an HTML file which you can examine later.
  

  <cftry>
  	<cfset session.userepid = REMATCH("\d{10}",CGI.SSL_CLIENT_S_DN_CN)[1] />
  	<cfcatch type="any">
  		<cfdump var="#CGI#" label="CGI dump: implies CGI.SSL_CLIENT_S_DN_CN does not contain ten-digit substring" format="html" output="#server.coldfusion.rootDir#\logs\CGI.SSL_CLIENT_S_DN_CN.dump.html">
  	</cfcatch>
  </cftry>
  ​

  
  Is cgi.SSL_CLIENT_S_DN_CN then what you expected
  
  
• Invalid date and datetime values
  The database may format dates and datetimes differently from your application. Even ColdFusion code based on different locales do represent dates differently. 
  So, I'd say the moral is: validate date values and datetime values before using them, especially if they're coming from another party
  Some relevant validation functions:
  IsDate
  IsValid
  LSIsDate
   

@WolfShade for your DateDiff issue, there is a breaking change introduced in CF2021 with date masks. The uppercase D now means day of year, instead of day of month. Change your date mask to lowercase to fix. Details here: https://www.petefreitag.com/blog/coldfusion-compatibility-scan/ 

Since you are already on CF2021, perhaps you had specified the JVM argument: -Dcoldfusion.datemask.useDasdayofmonth=true to override this behaviour.