First analysis : only version prior to 151014 are affected

Crosscheck was off by default for those old versions.

Confirm fix.
Was able to update to Version 2.06lts Build 160524.

On the first update you'll get header garbage infos inside the html page.
Which was the case when trying the first step on LS 2.5. So not introduced with this fix.

The text inserted above the confort key entry contained e.g. "X-Generator: GlotPress 0.1". Screenshot available, but not able to attach (since ticket is closed).

Update: The Note reopened the ticket. Screenshot ins uploaded. No issues while updating, just cosmetic.

That's weird. Needs to be checked why that text appears.

I explained in detail why in the previous comment ;-)
Cross check was disabled by default on those versions.
Now it's ok (understood, fixed, and tested, not a blind fix).

And if we speak about the weird garbage text on 150731: best way to solve it is to change the last build available via comfortUpdate to 150731 to something else (like 151014)

Carsten was talking about the garbage text.
What is meant by "to change" the last build available? Change by whom?

Still a long way till comfort is everywhere in the comfort updater.
The behavior is quite surprising. E.g.
https://www.limesurvey.org/forum/comfortupdate/105749-upgradeissue-alias-application-controllers-admin-update-is-invalid

jelo: all those bugs come from the old comfortUpdate, when you update from 2.05 to 2.06, you're using the old comfortupdate.

Changing the last build available: on the old comfortUpdate server.

I would expect that when the old comfortupdater is having the chance to update once via the comfortupdater it would get a fixed comfortupdater.
That seems not be the case since it updates the comfortupdater once and than got stuck.

Let me put my dumb user hat on: What is recommendation for him, the dumb user?
I want to update via comfortupdater. What is the next step for this dumb user. The others get LTS via GIT and don't look back in anger ;-)

we recommend to the d user to update regularly, and if he doesn't, to expect to encounter bugs that has been already fixed in recent versions.