Paladiac, one of the features of the current event is that you get event tickets from the daily quests. So, if you want to wait for a specific daily special prize (e.g. the Epic Wildlife 2023 selection kit), it makes a lot of sense to NOT complete your daily quests until the last day, then rush through them to get more minigames, i.e. more chances to win that specific daily special. This is why SOME of us had not already completed the event questline.
Just to confirm, I'm having the same issue (as Gobbi) with a blank questgiver on en9 and en11 (English international) worlds. Unable to complete last quest (q51). [On my D world, I already completed quest 51 so am not affected by the bug there.] Rebooting and using different browsers does not change anything. (I have tried installing and using Edge, Chrome, Firefox, and Brave, i.e. playing without addons does not make any difference either). I also tried simply doing the quest conditions (capturing 3 sectors), but did not get any progress - stuck at 50/51 on the event questline.
This issue is also affecting some of my guildmates. One tested on a tablet, it was broken there too, so appears to be affecting (some?) mobile users too, not just PC. Another guildmate is stuck at quest 49/51, also with a blank questgiver.
This appears to be another bug in today's update 1.257.
If anyone finds a successful workaround, I would love to read about it, thank you.