The bigger interfaces dampen the switch-on crackling not only on phones, but also on analog outputs.
There was information on forum and from RME, so I added this information some time ago to my interface comparison Excel.

https://www.tonstudio-forum.de/blog/ind … B-MADIfac/
Direct link to Excel: https://www.tonstudio-forum.de/index.ph … 0-08-xlsx/
See there at line 82: "Damping of the switch-on crackling"

So if this should happen "of all sudden" now without having anything else changed in your TM FX settings (preferences, ref level or fader settings, ..) then this might be a HW defect.
Therefore I would write an e-mail to support and ask whether repair is advised. I would do this quick as you said that you have the device since about 1y (maybe it's shortly before expiration of warranty).

However, it could also be due to somewhat unfortunate selected monitor and/or TM FX settings:

Other possible solutions

1. Monitor settings

Check whether your monitors have either a poti or switch to lower either the volume or to change sensitivity of inputs.
Choose the lowest volume level possible and check whether this already helps at 0dB volume.

2. Adjust TM FX settings

2.1  Set all HW outputs which have monitors / phones connected to the lowest reference level
       Positive side effect: then you can set faders closer to 0dB in the next stop 2.2 which results in higher SNR
2.1.1 - some analog outputs support up to 4 different reference levels, choose the one with lowest volume
2.1.2 - most phones outputs support up to 2 different reference levels, choose the one with lowest volume
2.2  Use Faders to set all HW outputs which have monitors / phones connected to a comfortably output level
2.3  Update these safe settings in all of your TotalMix FX Snapshots
2.4  If you do not use a snapshot, then mute all outputs, set it to save levels and save it to this "safe state" (not that you have volume jumps to 0dB or of you unmute by accident additionally)
2.5  In TotalMix FX Preferences activate two settings
2.5.1 That TotalMix FX restores the volume when recalling a Snapshot
         -> TM FX Preferences (F2) -> "Snapshots" -> Do not load "--"
2.5.2   That the recording interface will be initialized with settings from TotalMix FX on power-on
          -> TM FX Preferences (F2) -> "Device Handling" -> [x] Always init DSP devices with TotalMix FX settings

3. Fix level mismatches by either using attenuators or monitor controllers

Check this very informative posting from Matthias Carstens here:  https://forum.rme-audio.de/viewtopic.php?id=25399

3.1 Fix by using two switchable attenuators like this one here for a very affordable price:
      https://www.thomann.de/de/jts_ma_123.htm 2 x €18,90
3.2 Fix this by using a monitor controller
      Alternatively you can use a monitor controller like this, there are also more expensive ones available
      with more functions and connection to phones: https://www.thomann.de/gb/mackie_big_knob_passive.htm

4. More "luxury solutions" to this topics

Using a monitor controller or even a dedicated D/A converter with such features like the RME ADI-2 DAC/Pro

4.1 I saw this one at a "Tonmeister in Düsseldorf", very nice indeed:
       https://www.thomann.de/gb/spl_2control_black.htm €605

4.2 For this price I personally would get an ADI-2 DAC FS, as this device has even much more to offer
      and also restores volume of connected monitors and phones very reliable to the last used settings:
      https://www.thomann.de/de/rme_adi_2_dac_fs.htm €966

4.3 IMHO the best solution to use the ADI-2 Pro model:
      https://www.thomann.de/de/rme_adi_2_pro … dition.htm €1739

Differences between ADI-2 DAC / Pro, see this sticky: https://forum.rme-audio.de/viewtopic.php?id=32506