404 error problem with permalink on “/%postname%/” in multisite

Hello all,
I’ve done a lot of research everywhere: no solution.
I’m running in WordpPress 5.1.1 with MySQL 5.5 and php 7.0

On the multisite (partially in test), all sub-sites have the same problem (including the main site).
Everything was working correctly until yesterday when the sites crashed due to a problem with a plugin. I deleted the plugin concerned and I recovered the access to the sites.
All were set with permalink set to %postname% except one with %year%/%monthnum%/%day%/%postname% (blog which is not the main site and works well with this permalink configuration).

I applied this solution found on the net:
Switch the permalinks to”?p=123″, save and test it: everything works.
Back to permalinks in %postname% : again 404 error on all pages.

The .htaccess :

Options -Indexes

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} -f 
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]
# END WordPress

# Cache
<IfModule pagespeed_module>
ModPagespeed on
ModPagespeedEnableFilters
extend_cache,combine_css,combine_javascript,collapse_whitespace,move_css_to_head
</IfModule>
<IfModule mod_deflate.c>
AddOutputFilterByType DEFLATE text/html text/plain text/xml application/xml application/xhtml+xml text/css text/javascript application/javascript application/x-javascript
</IfModule>
# End Cache

# BEGIN Expire headers
<IfModule mod_expires.c>
ExpiresActive On
ExpiresDefault "access plus 500 seconds"
ExpiresByType image/x-icon "access plus 2500000 seconds"
ExpiresByType image/jpeg "access plus 2500000 seconds"
ExpiresByType image/png "access plus 2500000 seconds"
ExpiresByType image/gif "access plus 2500000 seconds"
ExpiresByType application/x-shockwave-flash "access plus 2500000 seconds"
ExpiresByType text/css "access plus 600000 seconds"
ExpiresByType text/javascript "access plus 600000 seconds"
ExpiresByType application/javascript "access plus 200000 seconds"
ExpiresByType application/x-javascript "access plus 200000 seconds"
ExpiresByType text/html "access plus 600 seconds"
ExpiresByType application/xhtml+xml "access plus 600 seconds"
</IfModule>
# END Expire headers

# BEGIN Cache-Control Headers
<IfModule mod_headers.c>
<filesMatch "\.(ico|jpe?g|png|gif|swf)$">
Header set Cache-Control "public"
</filesMatch>
<filesMatch "\.(css)$">
Header set Cache-Control "public"
</filesMatch>
<filesMatch "\.(js)$">
Header set Cache-Control "private"
</filesMatch>
<filesMatch "\.(x?html?|php)$">
Header set Cache-Control "private, must-revalidate"
</filesMatch>
</IfModule>
# END Cache-Control Headers

# 	HTTP Strict Transport Security
<IfModule mod_headers.c>
Header set Strict-Transport-Security "max-age=10886400; includeSubDomains; preload" env=HTTPS
Header set X-XSS-Protection "1; mode=block"
Header set X-Content-Type-Options nosniff
</IfModule>

# Wordfence WAF
<Files ".user.ini">
<IfModule mod_authz_core.c>
	Require all denied
</IfModule>
<IfModule !mod_authz_core.c>
	Order deny,allow
	Deny from all
</IfModule>
</Files>

# END Wordfence WAF

The plugin I removed: Woody ad snippets which is guaranteed up to WordPress 5.0.4

I remind you that everything worked perfectly until the crash caused by the simple access to the plugin in the administration interface (=> error 500 on the whole site, isolation of the plugin concerned and its removal).
Contact taken with my provider (OVH): no change or problem with them.

An idea of what is broken (in the DB ?, Elsewhere?)
Thanks in advance for your help !

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