In case the original poster checks back on this 8 months later, or anyone else has the same problem:
I had the same problem, and was able to hard-reset my UE Mini Boom by switching it on, and holding down the 'Bluetooth' and 'Volume Down' buttons.
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After 5–10 seconds the speaker made a sound effect (different from its usual on/off/pairing noises). I then made my iPhone 'forget' the speaker (Settings > Bluetooth > My Devices > UE MINI BOOM > "i" symbol > Forget This Device) and was then able to re-pair it as a new speaker.
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After 5–10 seconds the speaker made a sound effect (different from its usual on/off/pairing noises). I then turned off the speaker using the switch at the back, turned it back on, made my iPhone 'forget' the speaker (Settings > Bluetooth > My Devices > UE MINI BOOM > "i" symbol > Forget This Device), and was then able to re-pair it as a new speaker.
In case the original poster checks back on this 8 months later, or anyone else has the same problem:
I had the same problem, and was able to hard-reset my UE Mini Boom by switching it on, and holding down the 'Bluetooth' and 'Volume Down' buttons.
After 5–10 seconds the speaker made a sound effect (different from its usual on/off/pairing noises). I then made my iPhone 'forget' the speaker (Settings > Bluetooth > My Devices > UE MINI BOOM > "i" symbol > Forget This Device) and was then able to re-pair it as a new speaker.