Lots of PHP errors after importing al old site

Hi there.
Im trying to open a backup that was made to a site that’s live, but the backup could be a little old, but I really need it to be restored to check some specific config that was there.
These are the steps Im taking:
1.-Install XAMPP local. PHP, MySQL, all working good
2.-Create one table and droping latest version of WP in htdocs.
3.-Access local site. No errors. Install All in One WP Migration and open .wpress file
4.-No errors yet. Import sucessfully, front page shows good, but backend gives hundreds of PHP errors, like there was a mismatch in versiones, complaining about parameters in most PHP commands, etc
5.-I wonder if old backup was using older PHP, then I redo all with XAMPP version 5.6.? and get the same error
6.-I wonder if the “Base” wordpress should match the WP version that’s in the backup, so I download an older copy of WP (trying to guess the time the website was created), v5.1.4, dump it to a fresh htdocs and flushed tables, start over with base WP install, plugin install, import backup, and again same errors.

Sample of these errors:
Warning: preg_match() expects parameter 2 to be string, array given in C:\xampp56\htdocs\wp-includes\class-wp-block-parser.php on line 418

Warning: strlen() expects parameter 1 to be string, array given in C:\xampp56\htdocs\wp-includes\class-wp-block-parser.php on line 489

Warning: preg_match() expects parameter 2 to be string, array given in C:\xampp56\htdocs\wp-includes\class-wp-block-parser.php on line 418

Warning: strlen() expects parameter 1 to be string, array given in C:\xampp56\htdocs\wp-includes\class-wp-block-parser.php on line 489

Warning: preg_match() expects parameter 2 to be string, array given in C:\xampp56\htdocs\wp-includes\class-wp-block-parser.php on line 418

Warning: strlen() expects parameter 1 to be string, array given in C:\xampp56\htdocs\wp-includes\class-wp-block-parser.php on line 489

Warning: preg_match() expects parameter 2 to be string, array given in C:\xampp56\htdocs\wp-includes\class-wp-block-parser.php on line 418

Warning: strlen() expects parameter 1 to be string, array given in C:\xampp56\htdocs\wp-includes\class-wp-block-parser.php on line 489

Warning: preg_match() expects parameter 2 to be string, array given in C:\xampp56\htdocs\wp-includes\class-wp-block-parser.php on line 418

Warning: strlen() expects parameter 1 to be string, array given in C:\xampp56\htdocs\wp-includes\class-wp-block-parser.php on line 489

Warning: preg_match() expects parameter 2 to be string, array given in C:\xampp56\htdocs\wp-includes\class-wp-block-parser.php on line 418

Warning: strlen() expects parameter 1 to be string, array given in C:\xampp56\htdocs\wp-includes\class-wp-block-parser.php on line 489

Warning: Cannot modify header information – headers already sent by (output started at C:\xampp56\htdocs\wp-includes\class-wp-block-parser.php:489) in C:\xampp56\htdocs\wp-includes\pluggable.php on line 1251

Warning: Cannot modify header information – headers already sent by (output started at C:\xampp56\htdocs\wp-includes\class-wp-block-parser.php:489) in C:\xampp56\htdocs\wp-includes\pluggable.php on line 1254

Anyone can figure out what I’m missing??
Thanks a lot!

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