Hello,
I have this same problem.
I have the seed written down and saved and have entered it many times but the next button remains greyed out.
I have restored this wallet before so i am sure this is the seed phrase! some of the words in my phrase are in that big master list, but others arent. It was Electrum that generated the seed, i am using version 3.3.6
Any help would be much appreciated.
Double check the words, the spelling has to be 100% correct and not a single word can be typed incorrectly.
If you used Electrum to create the seed make sure that you don't select BIP39 from the the options menu.