Hi there,
I know this topic is old. I just created an account to share my insight and, as I mean no harm, to summarize the possible ways of approach.
With all due respect - I get immense help from places like this forum and I cannot thank you enough for being helpful, but 10 months later…
I did try to use the search function to find a solution to my similar problem.
And I found this solved topic. Well the solution is: use the search function, which I already did, so here I am with a solved problem, without a solution 
In old times when I used owncloud, I could limit the usage to my LAN, and it was enough for me. Time goes by and now I have to expose my nextcloud to Internet in order to be accessible at all (also from LAN). I am aware of bruteforce protection - the error seems to be self explanatory: too many tries.
What I (and possibly other people looking for an answer) need is either:
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a) to overcome it once, so I can put a right password, get accepted and forget or
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b) to disable bruteforce protection for my LAN.
I was actually searching for a).
Thank you @bb77 for pointing me to b). I will definitely check this out.
As for a) I will check things out, according to following posts from @jotatr and @michuvon