I have the 'Block Ads' setting checked, but I also have your site whitelisted. Unchecking the 'Block Ads' globally does not seem to alter the appearance of the notification.
I use the uBlock Origin extension, but I have it set to disable itself when I'm on www.tukui.org.
I use the Privacy Badger extension, but I have it set to disable itself when I'm on www.tukui.org.
Switching to Firefox (with the same Privacy Badger and uBlock Origin extensions (called add-ons there)) with both disabled for your site produces the same result.
I use Bitdefender as my anti-virus/anti-malware/anti-ransomware software, but it has not reported having blocked any content from your page (and it shows me a weekly report of every time it has protected me from anything - if it were doing this, it would brag about it).
The notification appears sometimes but not always. Sometimes I get ads (I'm fine with ads, by the way - yours anyway - I just get tired of being inundated with them everywhere else).
When it appears it shows as a graphical element near the bottom center of the page with no directly selectable text.
This is a cropped screenshot showing the notice:
"...enabled, If..." - should have a period after enabled or should (if you were really wanting a comma-spliced sentence) have the word "if" in all lower-case letters.
"ad blocker" vs "adblocker" - those may both be colloquially-acceptable ways to spell that, but within one paragraph it is an error to spell it inconsistently.
"...whitelist tukui.org..." - if the host were generating this, I would expect either the full name and domain portions of the URL to be used (www.tukui.org) or simply "this site" to appear here. In any case, there is no valid reason for abbreviating the URL in this context (the text is not crowded in its context and there are some situations where an incomplete or abbreviated name and domain will not work).
"...on your adblocker..." - standard usage here is "in" not "on" a software package, extension setting, or software setting.
Twenty-seven words, three punctuation marks, four errors. That's a functional literacy score lower than most fraudulent Indian-IRS-scam call centers.
I have difficulty believing this is being independently and uncontrollably generated by the host because the grammar, spelling, and usage errors are not indicative of a professional organization and there are two different references to your organization in two different formats (they might be picking that up from some site-registration information, but if they are, they'd also likely be picking up the full name/domain combination).
My concern is that this is some sort of hack against your website.
You folks lost your prior website not that long back and I doubt that whoever felt compelled to do that once simply gave up when you recovered (beautifully and with style) and moved here. The concern, at least in the abstract, seems valid.
Lacking administrator-level access to your site, however there is simply no way for me to find out what's going on except to bring it to your attention.
FYI - just during the time it took me to type this, the ads have reappeared and then disappeared again. The ad that reappeared was for winzipdriverupdater.com.
That happened when I alt-tabbed out to verify that I'm getting the same behavior out of Firefox.
No changes to my settings on this browser (Opera) at all - just alt-tabbed out and back - ads - alt-tabbed out and back to check my email - no ads.
If this isn't a hack, if that ungrammatical notice is indeed coming from your host, you might have a word with them about it and if they're basing your ad revenue on their ability to determine whether or not a given client is blocking ads or not, you might have a word with them about that as well.