Thank you for support.
I made all updates for Joomla and CB.
I manage all users with community builder, not with Joomla core. The issue was not directly my fault. 174 users was not listed in the Joomla user management, but they was still in the database. I checked the database and found an recurrently error: I had strange strings in "xxx_users" the column "params":
{"language":"","editor":"","timezone":"Europe\/Copenhagen","0":"{\"language\"
"\",\"editor\"
"\",\"timezone\"
"Europe\\\/Copenhagen\",\"0\"
"{\\\"language\\\"
\\"\\\",\\\"editor\\\"
\\"\\\",\\\"timezone\\\"
\\"Europe\\\\\\\/Copenhagen\\\",\\\"0\\\"
\\"{\\\\\\\"language\\\\\\\"
\\\\\\"\\\\\\\",\\\\\\\"editor\\\\\\\"
\\\\\\"\\\\\\\",\\\\\\\"timezone\\\\\\\"
\\\\\\"\\\\\\\"}\\\"}\"}","admin_style":"","admin_language":"","helpsite":""}
I solved this with a SQL code: UPDATE `xxx_users` SET `params` = '{}'
After cleaning the params the user in Joomla core was back again.
I know, this is not an CB issue. But do you have an idea why this issue comes over and over again? Updates had no effect.