Hello,
I’m trying to install LineageOS 18.1 on my FP3+ following instructions from this forum and (mostly consistent) the LineageOS wiki.
bootloader is unlocked, LineageOS recovery is flashed
lineage-18.1-20211220-nightly-FP3-signed.zip downloaded and checked for integrity (hash code and also this LOS “Verify Build Authenticity” was successful)
adb devices says: serial no. xxx recovery (I always need to enable adb on the FP in recovery=> advanced).
Running adb sideload performs the zip download up to 47%.
Then the FP interrupts installation with message “signature verification failed”, error 21
continue installation anyway => yes.
Then installation step1/2 starts, running 20-30s and aborting with error 29:
error applying update : 29 (kdownloadoperationhashmismatch)
error in /sideload/package.zip (status 1)
(adbd status 10)
I found an error log containing these error messages (among many info/some warnings):
191.240118: update_engine_sideload E 01-25 07:31:22 513
error:delta_performer.cc(1853) hash verification failed for operation 271 expected hash =
191.240438
error:delta_performer.cc(1856) calculated hash over 698272 bytes at offset: 182313237 =
191.240677
error:delta_performer.cc(707) mandatory operation hash check failed
191.240738
error:download_action.cc(336) error errorcode::kdownloadoperationhashmismatch (29)
in deltaperformer's write method when processing the received payload -- terminating
process
192.797055
error:prefs.cc(85) storage_->deletekey(key) failed.
(Also tried TWRP once by booting, not flashing it. But with this one try the download aborted after 10%)
Any help what goes wrong here would be appreciated very much 
Thanks
) doesn’t mean that it doesn’t contain errors, it just means that the file you downloaded is the same as the one on the server that enabled the hash code to be generated. I haven’t really looked round Lineage, but I see that there’s a pretty frequent update rate (weekly?) - which is all to their credit, but wonder how much testing gets done.