hundreds of session files created im tmp after update

After last update (1.7.6 through respoitory for Debian 10) hundreds of session files are created EVERY second in tmp directory, also there are several notices in error log:

No request delivery notification has been received from LSAPI application, possible dead lock

I would appreciate your help
downgrading didn't help, changing from lsphp74 to lsphp73 didn't help either.
changed to debian php-fpm 7.3 seems to work but quite slow.
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