It seems the latest Party update screw things up. It could be that I'm tired, but I am almost positive that the client changed button names for Open and Join when I restarted the client a couple of times. The Join button changed from AfxWnd4216 to AfxWnd4214, and I got the same error as you. Now I can't replicate that behaviour though, and the script works ok.
Try using exactly the same settings below as I did and see if things work.
Adde
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; EDIT THESE SETTINGS
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; Set what hotkeys should join tables.
; Leave empty (=) if you don't want to use a hotkey for a specific function.
; List of available hotkeys:
http://www.autohotkey.com/docs/KeyList.htm
pst_hotkey_join_min = F1
pst_hotkey_join_mid = F2
pst_hotkey_join_max = F3
; Set what hotkey should stop the script.
pst_hotkey_stop = F4
; Set if ANY table (with min players) should be joined (1) or not (0).
; If not, all full tables in current table list in lobby will be joined.
; Use this method to join only 6-max tables.
pst_method_is_any_table = 0
; Set how many tables should be joined for min/mid/max.
pst_min_tables = 1
pst_mid_tables = 6
pst_max_tables = 12
; THESE SETTINGS ONLY AFFECT JOINING "ANY" TABLE (NOT "FULL")
; Set what game type to join. MUST be exactly as shown in Partypoker lobby!
; Set blinds for level to join. MUST be exactly as shown in lobby table list.
; Set min players for tables to join.
pst_game_type = Limit Hold'em
pst_game_blinds = $5/$10
pst_min_players = 7
; Set num of seats for table to join. Should be 10 (full), 6 (short) or 2 (headsup).
pst_seats = 6
; Set how much, in percent, of default amount (as shown in Buy-in dialog) you should byuin for.
; If percentage buyin doesn't work, you can set an absoulte amount to buyin with instead.
; pst_buyin_amount is is used if greater than 0, otherwise percentage buyin will be used.
pst_buyin_percent = 100
pst_buyin_amount = 0
; Set if you should sitout (1) or not (0) when you join a new table.
pst_sitout = 1
; Set position where new tables should be moved to. Leave blank for no move.
; To position each new table at a different position, use setting (pst_dynamic_position) below.
pst_abs_position_x = 2395
pst_abs_position_y = 0
; Set if each new table should be positioned at a new location.
; Default setting below will position tables evenly across two monitors.
; NOTE: If you join 5 tables, they will be positioned according to the setting below, but if
; you later choose to join 5 new tables, the positioning will start over from the first
; coordinate given in this setting.
; pst_dynamic_position = 0.0/0.575/795.0/795.575/1600.0/1600.575/2395.0/2395.575/397.288/1997.288
; Set if tables should be joined from the current table list in lobby (1) or not (0).
; If not, script will navigate to tables according to game type and blinds settings.
; If script fails to navigate to chosen game type, set this to (1) and navigate
; to game type yourself before starting to join tables.
pst_current_table_list = 1
; Set how many full table waitlists that should be joined.
; This setting only tells how many waitlists should be joined for full tables, and not how
; many actual tables that will be joined.
pst_max_waitlists = 999
; Set if nonfull tables should be joined (1) or not (0) when joining waitlists for full tables.
pst_join_nonfull = 1
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; DO NOT EDIT BELOW HERE
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