This bug exists since the new clan system was added.
Basically the game still count the players that you had before the clan system change and thinks the limit is exceeded if you try to add more.
There is however a workaround by just asking the player to search for the clan through the "club information" page (not the ranking list) and apply for it manually. Deleting players will also "free space" but that is probably not what you want. The workaround will always work until you reach the 20 players limit.
There is no workaround for this, i have an empty clan (only master) and im not able to invite anyone
And it's not since "the new clan system was added"
It happened after the shity updates we've been getting for more than a year now.