Getting an error 500 (PHP memory allocation) when adding event participant
It might be that the pdf receipt format is too complex, try to disable sending them, see if it makes a difference

It's under admin > setting > misc

"Attach PDF copy to receipts"

If you need to send the pdf, I recommend you to let civi use a dedicated software to generate the pdf instead of the default (doing it in php, that uses tons of memory), it's a setting in the same page:
"Path to wkhtmltopdf executable"

you'll need to install this software on your server.