2006 Sprinter 2500 Starting Issue: Can anyone help PLEASE? And..Does dash lights flashing sequence provide diagnostic help?

208,000 miles on well-maintained vehicle. Over past 6 months, after driving vehicle for awhile (1hr+) and turn van off. wait..turn key, nothing. Pull key out, in, Turn key, Key out,in,turn a few more times, a few dash lights would come on, Key out,in,turn & all dash lights would come on, Turn key a few more times, starts fine. Check Eng light on (start error code).  The issue has gotten worse (more frequent). I replaced battery, same issues one month later. Dealer replaced Alternator (and wanted to replace battery again-battery tested fine, so not replaced). It seems to happen more often after driving in wet conditions and sitting over night. Now, when starting all dash lights come on, then flash (including the interior dome light) with a tac-tac-tac sound (the headlights are bright if I turn them on). If I disconnect and reconnect the battery terminals, the van starts up with no problem (except the check eng light is on) UNTIL after the next wet day/night.  Check eng light shows start error code. I took video of problem not starting van (dash lights flashing sequence (some lights flashed at different intervals) with tac-tac-tac sound),  Does the flashing sequence provide any diagnostic help?  PLEASE HELP!!!