Alternate Content and Footer: Background Image Address

Hi everybody,

I got a heavily Church Theme based website at https://freunde-bspg.de. Since the update to Enfold 4.5.6, the background images in the alternate content as well as in the footer area went lost. As you can see in the site analysis

.alternate_color {
background: #a6986f url('https://freunde-bspg.de/freunde-bspg.de/wp-content/uploads/2018/12/Solnhofen_Tile_alpha50-300x300.png') top center repeat scroll;
}

that the domain part of the background image’s address is somehow being doubled. The correct and working address (that is of course entered and also visible in the styling area of the backend) would be https://freunde-bspg.de/wp-content/uploads/2018/12/Solnhofen_Tile_alpha50-300×300.png.

Any hint where this is being generated or how I could get rid of this problem? Thanks in advance for any helping hints!