That error isn't present in your current debug log.
If that occurs again, let's see how many DNS requests Pi-hole had to handle in the minute before, and whether that would be related to certain domains or clients:
sudo pihole-FTL sqlite3 /etc/pihole/pihole-FTL.db "SELECT count(*) FROM queries \
WHERE timestamp > strftime('%s','2025-03-18 15:00:00.142', '-60 seconds', 'utc') \
AND timestamp <= strftime('%s','2025-03-18 15:00:00.142', 'utc');"
sudo pihole-FTL sqlite3 /etc/pihole/pihole-FTL.db "SELECT domain, count(*) FROM queries \
WHERE timestamp > strftime('%s','2025-03-18 15:00:00.142', '-60 seconds', 'utc') \
AND timestamp <= strftime('%s','2025-03-18 15:00:00.142', 'utc') \
GROUP BY domain ORDER BY 2 DESC LIMIT 10;"
sudo pihole-FTL sqlite3 /etc/pihole/pihole-FTL.db "SELECT client, count(*) FROM queries \
WHERE timestamp > strftime('%s','2025-03-18 15:00:00.142', '-60 seconds', 'utc') \
AND timestamp <= strftime('%s','2025-03-18 15:00:00.142', 'utc') \
GROUP BY client ORDER BY 2 DESC LIMIT 10;"
Substitute 2025-03-18 15:00:00.142 with the timestamp as logged in Pi-hole's error message.
Maximum concurrent warnings are often caused by a DNS loop of sorts, or by upstream DNS server unresponsiveness, less often by a misbehaving client issuing DNS queries excessively.
It seems there are at least three router type devices in your network, as your debug log shows three different RAs, originating from your Fritzbox, a Draytek and an unknown device.
Does anyone of those use your Pi-hole for DNS server?
Also, your debug log shows that your Fritzbox is advertising its own IPv6 addresses as local DNS servers:
*** [ DIAGNOSING ]: contents of /etc
-rw-r--r-- 1 root root 151 Mar 16 06:52 /etc/resolv.conf
nameserver 192.168.104.39
nameserver fd<redacted>89
nameserver 2a<redacted>89
Fritzbox routers can be configured to not advertise any IPv6 DNS servers at all, see Unresolved ipv6 adress in my top list - #4 by Bucking_Horn.