After adding a new member and a registration profile field and deleting them both in beta 6, when I try to sign-up a new member it generates Error code EX1064. Clearing cache doesn't help.

SELECT * FROM `core_pfields_content` WHERE member_id = IS NULL
IPS\Db\Exception: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'IS NULL' at line 1 (1064)
#0 /home/vii/public_html/system/Db/Select.php(402): IPS\Db->preparedQuery()
#1 /home/vii/public_html/system/Db/Select.php(460): IPS\Db\Select->runQuery()
#2 /home/vii/public_html/system/Db/Select.php(384): IPS\Db\Select->rewind()
#3 /home/vii/public_html/applications/core/extensions/core/ProfileSteps/ProfileFields.php(250): IPS\Db\Select->first()
#4 /home/vii/public_html/applications/core/modules/front/system/register.php(257): IPS\core\extensions\core\ProfileSteps\ProfileFields::wizard()
#5 /home/vii/public_html/system/Dispatcher/Controller.php(139): IPS\core\modules\front\system\register->manage()
#6 /home/vii/public_html/system/Dispatcher/Dispatcher.php(169): IPS\Dispatcher\Controller->execute()
#7 /home/vii/public_html/index.php(16): IPS\Dispatcher->run()
#8 {main}

I corrected the error/problem by deleting the new profile field I had added, but I suppose the problem could arise again.

There is no license associated with this account, and I am unable to replicate this. Please ensure you have this email added as an alternative contact if your license is under another email address

i believe the email account association is straightened out now.

regarding the error, after i created a new custom profile field, my admin section support area showed a Mariadb error that might be related to the prior error:

ALTER TABLE `core_pfields_content` ADD COLUMN `field_1` MEDIUMTEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NULL ;

It was "automatically fixed." Are there any know compatibility issues with MariaDB vs MySQL?

Im not aware of any specifically, but we recommend MySQL. We're specifically optmising on MySQL 8 if that helps.