For one specific user, it is unable to impersonate even for admin.
Login as admin user.
Then try to impersonate as specific user. Page is redirected to blank page.
Default-thread-15 38ECB2113727D34028C15AA543990EBD txid=f1fc72113727 Impersonate: Impersonation start: XXXXXX (C2782949 )
Default-thread-15 38ECB2113727D34028C15AA543990EBD txid=f1fc72113727 *** End #152934 /api/now/ui/impersonate/c5550986db0e13046ad01cd74b961931, user: C2782949
Default-thread-1 SYSTEM txid=71fc72113727 WARNING *** WARNING *** Unwanted leading or trailing whitespace for user=C2782949
All Supported ServiceNow Releases.
user_name field value had a extra blank space.
e.g. logs shows >>> Impersonate: Impersonation start: XXXXXX (C2782949 ) >> there is a extra space after username and brackets.
After removing the extra blank (trailing whitespace), impersonation issue was resolved.