After searching for a bit, i noticed that the same exact problem you're having was already reported (quite recently too) on the official gitlab repo of remmina. Go here if you want to see what i mean.
Now for the possible cause:
As written on the post i linked above, the error/problem you're facing might stem from a newer version of FreeRDP, not Remmina (as stated by him on the aforementioned post)
It comes from an older version of remmina that wasn't removed entirely after an upgrade.(as also stated on the very last comment on the aforementioned link)
Since you're on Linux Mint, i suggest doing as the last comment on the post recommended, and completely purge both remmina and FreeRDP and then install it.
sudo apt purge -y "?name(^remmina.*)" "?name(^libfreerdp.*)" "?name(^freerdp.*)" "?name(^libwinpr.*)" && sudo apt install -y "?name(^remmina.*)" "?name(^libfreerdp.*)" "?name(^freerdp.*)" "?name(^libwinpr.*)"