Found it! 😊 Although I still don't know where this was coming from.ย 🤨ย
There was a .htaccess file in the wp-includes folder, with the following content:ย
Deny from all
<FilesMatch "\.(html|HTML|htm|HTM|xhtml|XHTML|js|JS|css|CSS|bmp|BMP|png|PNG|gif|GIF|jpg|JPG|jpeg|JPEG|ico|ICO|pcx|PCX|tif|TIF|tiff|TIFF|au|AU|mid|MID|midi|MIDI|mpa|MPA|mp3|MP3|ogg|OGG|m4a|M4A|ra|RA|wma|WMA|wav|WAV|cda|CDA|avi|AVI|mpg|MPG|mpeg|MPEG|asf|ASF|wmv|WMV|m4v|M4V|mov|MOV|mkv|MKV|mp4|MP4|swf|SWF|flv|FLV|ram|RAM|rm|RM|doc|DOC|docx|DOCX|txt|TXT|rtf|RTF|xls|XLS|xlsx|XLSX|pages|PAGES|ppt|PPT|pptx|PPTX|pps|PPS|csv|CSV|cab|CAB|arj|ARJ|tar|TAR|zip|ZIP|zipx|ZIPX|sit|SIT|sitx|SITX|gz|GZ|tgz|TGZ|bz2|BZ2|ace|ACE|arc|ARC|pkg|PKG|dmg|DMG|hqx|HQX|jar|JAR|xml|XML|pdf|PDF|([s,S][q,Q][l,L])|([j,J][p,P][g,G])|([g,G][i,I][f,F])|([p,P][n,N][g,G])|([t,T][x,X][t,T])|([d,D][o,O][c,C])|([d,D][o,O][c,C][x,X])|([x,X][l,L][s,S])|([x,X][l,L][s,S][l,L])|([p,P][d,D][f,F])|([r,R][a,A][r,R])|([x,X][m,M][l,L])|([v,V][s,S][d,D])|([v,V][d,D][x,X])|([p,P][p,P][t,T])|([p,P][p,P][t,T][x,X])|([s,S][q,Q][l,L])|([f,F][l,L][v,V])|([s,S][w,W][f,F])|([m,M][p,P][1,9]))$">
Allow from all
</FilesMatch>
No idea how this file could land there. But as soon as it was gone, the icons were back, and there was no error in the console anymore.ย ย 🥂ย