I had this too:
[16-Aug-2020 10:08:00 UTC] PHP Warning: Class ‘Tightenco\Collect\Support\LazyCollection’ not found in …/vendor/tightenco/collect/src/Collect/Support/alias.php on line 19
[16-Aug-2020 10:08:00 UTC] PHP Parse error: syntax error, unexpected ‘=’ in …/vendor/tightenco/collect/src/Collect/Support/LazyCollection.php on line 611
It was because MAUTIC requires PHP 7.2 and even though I had upgraded an htaccess setting had it at 7.0 still.
I also got a memory error after that was fixed:
[16-Aug-2020 10:53:07 UTC] PHP Fatal error: Allowed memory size of 33554432 bytes exhausted (tried to allocate 32768 bytes) in /home/jewishdata/public_html/newsletter/app/bundles/LeadBundle/Model/CompanyModel.php on line 367
[16-Aug-2020 10:53:07 UTC] PHP Fatal error: Allowed memory size of 33554432 bytes exhausted (tried to allocate 32768 bytes) in /home/jewishdata/public_html/newsletter/vendor/psr/log/Psr/Log/LogLevel.php on line 1
Which we fixed by increasing the allotted memory via an ini file.