I have applied a temporary fix for the participants token import/editing error.
In the file application/models/Token.php, I commented out lines 318 and 319 and added an entry.
My code now looks like this:

    // According to Yii doc : http://www.yiiframework.com/doc/api/1.1/CSecurityManager#generateRandomString-detail

$pattern = Yii::app()->getConfig("allowedcharacters_pattern_token");

return preg_replace($pattern, '', $token);

    return preg_replace('/[^A-Za-z0-9_]/', '', (string)$token);

I am awaiting the official fix.

A similar issue (20587) was already closed before I could comment, so I am posting my comment here.
The problem was on my end because, during manual upgrades, I always copied the old config-defaults.php file along with config.php and security.php, so I didn't have the latest changes.
Thanks to Haythem for the advice below :
2026-07-22 09:57
developer 20587:85238
Hi,
Most likely you're missing a config in your application/config/config-defaults.php

$config['allowedcharacters_patterntoken'] = '/[^0-9a-zA-Z-~]/';
If it's not there, just add it, that's most likely what's causing the empty token issue. Let us know how it goes !

You can close my issue.