PS : it happen with a copied survey : seems this settings is set 'Open' by default.
Unsure what happen when update

Just have a urgent call by a prospect about this issue.

By default.. INSERT a record ommitting access_mode will result in access_mode = 'O'

A closer look... access_mode seems obsolete. It is not used in the code anymore.

We had all sorts of issues with tokens in the most recent versions
We went back to 6.15.1+250625 and all is well again! It works like a charm!

I just test on https://shnoulle.limesurvey.net

If you leave Open access mode when activate :

Even with token table : you can enter survey
https://shnoulle.limesurvey.net/327729?newtest=Y
https://shnoulle.limesurvey.net/327729?token=TOKENTEST&newtest=Y

I noticed just now... in lime_survey table 'access_mode' ('O' default or 'C') is introduced in 6.15.7.
So it is brand new. But I think this setting is not very good implemented yet.

For us one thing is very clear... Token management is faulty in 6.15.7 and 1 or 2 versions before that.

token management:
6.15.4 is OK
6.15.5 onward Fails

correction 6.15.4 is also at fault...
6.15.3 is OK

As of 5.16.4 this form is not shown anymore (restore old tokens)
So for us 5.16.3 is the fixed version. Until tokenmanagement is completely fixed

Schermafbeelding 2025-08-25 112534.png (60,956 bytes)   

Schermafbeelding 2025-08-25 112534.png (60,956 bytes)   

correction...
As of 6.16.4 this form is not shown anymore (restore old tokens)
So for us 6.16.3 is the fixed version. Until tokenmanagement is completely fixed

see also https://bugs.limesurvey.org/view.php?id=20235

For what it is worth:
IMHO: https://github.com/LimeSurvey/LimeSurvey/pull/4300 is causing the issues