Unable to complete this quest.

Had cleared Cragslane Cavern before entering Riften.
Had not entered Riften until player was at higher level (Player at approx Level 30).

Following the Jarl of Riften’s request to “Disrupt the skooma operation” led player to re-enter Cragslane Cavern.
Bandit Chief had respawned, and I was able to defeat again. However, did not get the message that this step in the quest was completed, neither did returning to speak to the Jarl of Riften offer any appropriate dialogue option.

Message “Disrupt the skooma operation” appears in Miscellaneous quest log, but can no longer be selected to identify map location.

Tried re-entering Cragslane Cavern 3rd and 4th time (Player is now Level 48). Each time Bandit Chief had re-spawned, but defeating the chief did not conclude the quest.

I had the same bug. Defeat Bandit 7 times and quest will be completed. Bandit respawns every 30 days so good luck and have patience:)

Any exploration = broken quests

Will try the 7 x route. Thanks for the help.

i have this problem too. it looks like the 7x route is not a defined answer to the solution. it just automagically met a condition that trigger the next event.