Import 'encountered an unexpected error' between Step 2 and 3 with 500 rows
Using 5.51.1 Import Contacts the error "The website encountered an unexpected error. Please try again later." This phrase occurs in the CiviCRM code base in several .php files, but is not a normal onscreen backtrace debug output and nothing giving more clues in Civi's log files that I can tell.
It occurs in this situation: a file of 500 rows x 30 columns after about 3 seconds of loading after pressing the button in Step 2 to continue to Step 3.
No error appears when that same file is split in half to only 250 rows.
I have tried this same full CSV file (500 rows) on a 5.50.4 install and there is no error.
The import for Individuals with typical data (name, email phone) but does contain several custom fields and an employer relationship. There are also some special characters (international names) I wonder if the import is not handling those well.
Although I cannot provide you my data I would be happy to test if you have a test bed and a way to generate similar testing data of this nature for an import. I hope this has been helpful.