Can you please provide a screenshot of that error?

Here is the screenshot. This is a test installation of LS with no surveys and no other users except for the admin.

FireShot Screen Capture #086 - 'LimeSurvey' - www_marketing-forge_com_LStest_index_php_admin_user_sa_adduser.png (31,236 bytes)   

FireShot Screen Capture #086 - 'LimeSurvey' - www_marketing-forge_com_LStest_index_php_admin_user_sa_adduser.png (31,236 bytes)   

What email address are you trying to enter?
Do you maybe put unintentionally any space characters in the input field?

I cannot reproduce this issue, so can nobody else. Maybe it is a bug in that particular PHP version, hard to say. Anyway, 2.6.x is out of support except for critical security vulnerabilities and this issue does not qualify, anyway.
Please update to the latest version instead.

Hi,
I try with several valid email addresses.
Definitely no blank spaces.
Tested on several PHP versions and the error keeps showing (even on PHP 7).
Right now an upgrade to the new LS is not an option - we use several custom templates which are not compatible and should be recoded.
I also have more than 100 users with more than 1000 surveys.
I will test the new version with this database in the summer and I will also need to schedule a seminar for the changes in the new version (the users are familiar with the old LS and there will be some transition time).

What if I try to add a new user directly in the database? I should be careful for the id of course. I will add parent id, username, full name and email address. Anything else to look for? Are there any potential problems with this approach?