Changing a person's group assignment throws an error, though change is accomplished

When I change a person’s group assignment, I am taken to a page which has (in very large text!) the following errors:

Warning Invalid argument supplied for foreach()

ee/EllisLab/ExpressionEngine/Controller/Members/Profile/Profile.php, line 229 show details

Severity: E_WARNING

Warning Cannot modify header information - headers already sent by (output started at /home/ruralint/public_html/risys/ee/legacy/core/Exceptions.php:112)

ee/legacy/core/Input.php, line 249 show details

Severity: E_WARNING

Warning Cannot modify header information - headers already sent by (output started at /home/ruralint/public_html/risys/ee/legacy/core/Exceptions.php:112)

ee/legacy/libraries/Functions.php, line 437 show details

Severity: E_WARNING

Any idea what is causing that and what I can do to allow changing group assignments not throw these errors?

Terry