I've no idea which addon/plugin is responsible. I was using aoe song buffs and the whole game started to lag badly and I was getting a spam of "Unable to cast spell." Tried unloading gearswap but that did nothing. After force DCing twice (happened again after reloading FFXI) I manually loaded a bunch of addons and plugins but I wasn't getting any repeated issues, so I'm at a loss to which one is the culprit. I'll give you a list of the ones I usually use (including plugin_manager):
<addon>aecho</addon> <addon>battlemod</addon> <addon>cancel</addon> <addon>chatPorter</addon> <addon>enternity</addon> <addon>eval</addon> <addon>gametime</addon> <addon>GearSwap</addon> <addon>ohShi</addon> <addon>pet_fix</addon> <addon>porter</addon> <addon>shortcuts</addon> <addon>thtracker</addon> <addon>Treasury</addon> <plugin>Attainment</plugin> <plugin>AutoExec</plugin> <plugin>BlinkMeNot</plugin> <plugin>BoxHelper</plugin> <plugin>Chatmon</plugin> <plugin>Clock</plugin> <plugin>DelayMeNot</plugin> <plugin>Distance</plugin> <plugin>DrawDistance</plugin> <plugin>FFXIDB</plugin> <plugin>Guildwork</plugin> <plugin>Infobar</plugin> <plugin>Itemizer</plugin> <plugin>LightLuggage</plugin> <plugin>PetTP</plugin> <plugin>Silence</plugin> <plugin>Timers</plugin> <plugin>Timestamp</plugin> <plugin>TParty</plugin>
Maybe if someone else playing bard using one of the above addons/plugins who gets the same issue could shed some light on which one could be responsible.