I was also suffering the same and clearing the browser cache works but I still see lots of worrying messages in the debug output.
I moved your post to a new topic, because the error in your log (I have no access to) might be unrelated to the other issue.
Thanks. Here is a section of it:
*** [ DIAGNOSING ]: contents of /var/log/lighttpd
-rw-r--r-- 1 www-data www-data 4335 Jul 16 07:46 /var/log/lighttpd/error.log
2020-07-13 06:25:07: (server.c.1534) logfiles cycled UID = 0 PID = 28835
2020-07-16 04:16:05: (server.c.1828) server stopped by UID = 0 PID = 1
2020-07-16 04:16:07: (log.c.217) server started
2020-07-16 04:16:07: (server.c.1295) WARNING: unknown config-key: alias.url (ignored)
2020-07-16 04:24:40: (mod_fastcgi.c.2543) FastCGI-stderr: PHP Warning: inet_pton(): Unrecognized address in /var/www/html/admin/settings.php on line 745
2020-07-16 04:25:05: (mod_fastcgi.c.2543) FastCGI-stderr: PHP Warning: inet_pton(): Unrecognized address in /var/www/html/admin/settings.php on line 745
2020-07-16 04:25:09: (server.c.1828) server stopped by UID = 0 PID = 1
2020-07-16 04:25:41: (log.c.217) server started
2020-07-16 04:25:41: (server.c.1295) WARNING: unknown config-key: alias.url (ignored)
2020-07-16 05:13:42: Not allowed (login session invalid or expired, please relogin on the Pi-hole dashboard)!
2020-07-16 05:14:24: Not allowed (login session invalid or expired, please relogin on the Pi-hole dashboard)!
2020-07-16 05:18:11: Not allowed (login session invalid or expired, please relogin on the Pi-hole dashboard)!
2020-07-16 05:32:03: (mod_fastcgi.c.2543) FastCGI-stderr: PHP Warning: inet_pton(): Unrecognized address in /var/www/html/admin/settings.php on line 745
2020-07-16 05:32:47: (mod_fastcgi.c.2543) FastCGI-stderr: PHP Warning: inet_pton(): Unrecognized address in /var/www/html/admin/settings.php on line 745
2020-07-16 05:36:15: (mod_fastcgi.c.2543) FastCGI-stderr: PHP Warning: inet_pton(): Unrecognized address in /var/www/html/admin/settings.php on line 745
2020-07-16 05:37:39: Not allowed (login session invalid or expired, please relogin on the Pi-hole dashboard)!
2020-07-16 05:41:22: (server.c.1828) server stopped by UID = 0 PID = 1
2020-07-16 05:41:54: (log.c.217) server started
2020-07-16 05:41:54: (server.c.1295) WARNING: unknown config-key: alias.url (ignored)
2020-07-16 05:46:03: Not allowed (login session invalid or expired, please relogin on the Pi-hole dashboard)!
2020-07-16 05:46:54: Not allowed (login session invalid or expired, please relogin on the Pi-hole dashboard)!
2020-07-16 06:12:03: (server.c.1828) server stopped by UID = 0 PID = 1
2020-07-16 06:12:05: (log.c.217) server started
2020-07-16 06:12:05: (server.c.1295) WARNING: unknown config-key: alias.url (ignored)
2020-07-16 06:23:28: (mod_fastcgi.c.2543) FastCGI-stderr: PHP Warning: inet_pton(): Unrecognized address in /var/www/html/admin/settings.php on line 745
Line 745 of settings.php is:
<td data-order="<?php echo bin2hex(inet_pton($lease["IP"])); ?>"><?php echo $lease["IP"]; ?></td>
Mhh... this line was modified 2 years ago...
https://github.com/pi-hole/AdminLTE/commit/60dde5314075494ba245785846afafbd31058cc8
Maybe one of your DHCP leases has a strange address (format)?
Are you using Pihole as DHCP server at all?
According to your debug log, Pi-hole's core is fully operational, with full IPv4 connectivity and a working web UI, the only exception being a block page mismatch:
*** [ DIAGNOSING ]: Dashboard and block page
[✗] Block page X-Header: X-Header does not match or could not be retrieved.
HTTP/1.1 200 OK
[✓] Web interface X-Header: X-Pi-hole: The Pi-hole Web interface is working!
This would be explained if you'd customised your block page or web UI somehow.
If you didn't:
There have been reports by numerous users (including me) observing varying obscure UI behaviour that evaporates the moment you clear your browser cache.
I'll keep my finger's crossed that it does the same for you. 
No, I haven’t customized anything and a browser refresh hasn’t fixed it.
Where do I find the block page normally? What is the file name?
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