Uncaught exception on session_write_close after upgrade to 8.5.3

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Since an upgrade 8.5.2->8.5.3 I now see these in my Apache error log:

[Fri Jun 05 13:27:44.482530 2020] [php7:error] [pid 107994] [client] PHP Fatal error:  Uncaught Exception: Serialization of 'Closure' is not allowed in [no active file]:0\nStack trace:\n#0 [internal function]: session_write_close()\n#1 {main}\n  thrown in [no active file] on line 0

I also see a growing number of running Apache processes, with the growth coinciding exactly with the time of the upgrade, so I suspect it's related. My Selenium-based prober started timing out regularly about 25% of the time, which I also believe is related.

Status: New
zagrodzki replied on at Permalink Reply
Also present after 8.5.4 upgrade.

concrete5 Environment Information

# concrete5 Version
Core Version - 8.5.3
Version Installed - 8.5.3
Database Version - 20200501000000

# Database Information
Version: 5.7.30-0ubuntu0.18.04.1

# concrete5 Packages
SimpWeb (0.1.9)

# concrete5 Overrides

# concrete5 Cache Settings
Block Cache - On
Overrides Cache - Off
Full Page Caching - On - If blocks on the particular page allow it.
Full Page Cache Lifetime - Every 6 hours (default setting).

# Server Software
Apache/2.4.29 (Ubuntu)

# Server API

# PHP Version

# PHP Extensions
apache2handler, calendar, Core, ctype, date, dom, exif, fileinfo, filter, ftp, gd, gettext, hash, iconv, json, libxml, mbstring, mysqli, mysqlnd, openssl, pcre, PDO, pdo_mysql, Phar, posix, readline, Reflection, session, shmop, SimpleXML, sockets, sodium, SPL, standard, sysvmsg, sysvsem, sysvshm, tokenizer, wddx, xml, xmlreader, xmlwriter, xsl, Zend OPcache, zlib

# PHP Settings
max_execution_time - 30
log_errors_max_len - 1024
max_file_uploads - 20
max_input_nesting_level - 64
max_input_time - 60
max_input_vars - 1000
memory_limit - 512M
post_max_size - 512M
upload_max_filesize - 512M
mysqli.max_links - Unlimited
mysqli.max_persistent - Unlimited
pcre.backtrack_limit - 1000000
pcre.recursion_limit - 100000
session.cache_limiter - <i>no value</i>
session.gc_maxlifetime - 7200
opcache.max_accelerated_files - 10000
opcache.max_file_size - 0
opcache.max_wasted_percentage - 5

Browser User-Agent String

Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.61 Safari/537.36