A reproducible example is a chunk of code everyone else can copy-paste and see the problem for themselves in their own development environment. Linking to some json files and your code might work, but it is also likely to have a lot more than is required to address this problem. That makes it more overhead for any reader.
E.g. A reproducible example
library(jsonlite)
json_txt <- '{"foo": [1,2,3], "bar": [1,2,3,4]}'
myvar <- fromJSON(json_txt, flatten=T)
myframe <- as.data.frame(myvar)
#Error in (function (..., row.names = NULL, check.rows = FALSE, check.names = TRUE, :
# arguments imply differing number of rows: 3, 4
Which gives the same error message you're seeing, includes a small sample of the data, and is self contained. Aside from installing jsonlite, a reader would not need to do anything other than copy-paste the example into their own development environment. Also, in making a reproducible example you might discover insight into your own problem.
Mismatched number of items in arrays
Incidentally, making the example of a reproducible example led to a discovery that might help you. It's likely that your data has arrays that aren't multiples of each other's lengths. This may be what's causing the conversion to data.frame to fail for you. In the above example if the lengths of foo and bar are multiples of each other, the conversion works. E.g. try the above with "foo":[1,2] in the json text.