Just like you, Marc, I cannot see any connection between ESCHER and LASHER. I don't see where the L should come from.
Still, there may be more to be said about the two surnames.
ESCHER:
- Habitional name from Middle High German esch(e), grain-field
- Someone from Esch(e/r)
- Occupational name from Middle High German escher, exhausted ashes, thus soap-boiler
GLEICHMANN:
- Nickname for a moderate or straight forward person or someone who seeks harmony
- Habitional name from plot names like "Auf der Gleiche" (On the Plain)
- From the placename Gleichen
Sources:
Rosa und Volker Kohlheim, Familiennamen, Duden Verlag Mannheim 2000
Grimms Deutsches Wörterbuch, "Gleiche"
http://germazope.uni-trier.de/Projects/WBB/woerterbuecher/dwb/wbgui?lemmode=lemmasearch&mode=hierarchy&textsize=600&onlist=&word=gleiche&lemid=GG18013&query_start=1&totalhits=0&textword=&locpattern=&textpattern=&lemmapattern=&verspattern=#GG18013L0