Hey Paul
I exported content from a dev site to a new local site.
On Import I adjusted a user and then added the content: I received the following error message (I do not know if it is the same as before so that I am then adding a new issue for it.)
Can not modify header information - headers already sent in /Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-includes/option.php on line 837
Call Stack
Time Memory Function Location
1 0.0016 397632 {main}( ) ../admin.php:0
2 0.2840 31799624 call_user_func:{/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-admin/admin.php:309} ( ) ../admin.php:309
3 0.2840 31799696 WXR_Import_UI->dispatch( ) ../admin.php:309
4 0.2840 31799968 WXR_Import_UI->display_import_step( ) ../class-wxr-import-ui.php:110
5 0.2931 31864808 require( '/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-content/plugins/wordpress-importer-v2-master/templates/import.php' ) ../class-wxr-import-ui.php:445
6 0.2942 31871440 WXR_Import_UI->render_header( ) ../import.php:41
7 0.2942 31873528 require( '/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-content/plugins/wordpress-importer-v2-master/templates/header.php' ) ../class-wxr-import-ui.php:119
8 0.2945 31913952 require_once( '/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-admin/admin-header.php' ) ../header.php:7
9 0.2950 31915952 wp_user_settings( ) ../admin-header.php:63
10 0.2955 31916600 setcookie ( ) ../option.php:837
( ! ) Warning: Cannot modify header information - headers already sent in /Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-includes/option.php on line 838
Call Stack
Time Memory Function Location
1 0.0016 397632 {main}( ) ../admin.php:0
2 0.2840 31799624 call_user_func:{/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-admin/admin.php:309} ( ) ../admin.php:309
3 0.2840 31799696 WXR_Import_UI->dispatch( ) ../admin.php:309
4 0.2840 31799968 WXR_Import_UI->display_import_step( ) ../class-wxr-import-ui.php:110
5 0.2931 31864808 require( '/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-content/plugins/wordpress-importer-v2-master/templates/import.php' ) ../class-wxr-import-ui.php:445
6 0.2942 31871440 WXR_Import_UI->render_header( ) ../import.php:41
7 0.2942 31873528 require( '/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-content/plugins/wordpress-importer-v2-master/templates/header.php' ) ../class-wxr-import-ui.php:119
8 0.2945 31913952 require_once( '/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-admin/admin-header.php' ) ../header.php:7
9 0.2950 31915952 wp_user_settings( ) ../admin-header.php:63
10 0.2972 31916656 setcookie ( ) ../option.php:838
A plugin such as WooCommerce is active on the dev site but not active on the local site.
Here is also a screenshot:

Hey Paul
I exported content from a dev site to a new local site.
On Import I adjusted a user and then added the content: I received the following error message (I do not know if it is the same as before so that I am then adding a new issue for it.)
Can not modify header information - headers already sent in /Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-includes/option.php on line 837
Call Stack
Time Memory Function Location
1 0.0016 397632 {main}( ) ../admin.php:0
2 0.2840 31799624 call_user_func:{/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-admin/admin.php:309} ( ) ../admin.php:309
3 0.2840 31799696 WXR_Import_UI->dispatch( ) ../admin.php:309
4 0.2840 31799968 WXR_Import_UI->display_import_step( ) ../class-wxr-import-ui.php:110
5 0.2931 31864808 require( '/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-content/plugins/wordpress-importer-v2-master/templates/import.php' ) ../class-wxr-import-ui.php:445
6 0.2942 31871440 WXR_Import_UI->render_header( ) ../import.php:41
7 0.2942 31873528 require( '/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-content/plugins/wordpress-importer-v2-master/templates/header.php' ) ../class-wxr-import-ui.php:119
8 0.2945 31913952 require_once( '/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-admin/admin-header.php' ) ../header.php:7
9 0.2950 31915952 wp_user_settings( ) ../admin-header.php:63
10 0.2955 31916600 setcookie ( ) ../option.php:837
( ! ) Warning: Cannot modify header information - headers already sent in /Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-includes/option.php on line 838
Call Stack
Time Memory Function Location
1 0.0016 397632 {main}( ) ../admin.php:0
2 0.2840 31799624 call_user_func:{/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-admin/admin.php:309} ( ) ../admin.php:309
3 0.2840 31799696 WXR_Import_UI->dispatch( ) ../admin.php:309
4 0.2840 31799968 WXR_Import_UI->display_import_step( ) ../class-wxr-import-ui.php:110
5 0.2931 31864808 require( '/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-content/plugins/wordpress-importer-v2-master/templates/import.php' ) ../class-wxr-import-ui.php:445
6 0.2942 31871440 WXR_Import_UI->render_header( ) ../import.php:41
7 0.2942 31873528 require( '/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-content/plugins/wordpress-importer-v2-master/templates/header.php' ) ../class-wxr-import-ui.php:119
8 0.2945 31913952 require_once( '/Users/joachim/Documents/Websites/kooperativet-sep2017.dev/wp-admin/admin-header.php' ) ../header.php:7
9 0.2950 31915952 wp_user_settings( ) ../admin-header.php:63
10 0.2972 31916656 setcookie ( ) ../option.php:838
A plugin such as WooCommerce is active on the dev site but not active on the local site.
Here is also a screenshot: