SRT-Subtiltes no longer displayed Kodi-nighlty

I am using the Kodi nightly on a Ubuntu16.04 machine, and since a couple of days I realized that for most (90%) of my movies subtitles (all in srt-format) are no longer found and displayed.

* Subtitles which displayed fine last week, are no longer shown
* To narrow to problem, I ran ‘Subtilte Edit’ (windows program) –> ‘fix common errors’ but it did not help
* Kodi-Logs don’t show anything related to subtiltes.
* Downgrading from ‘Nightly’ (Kodi 18) to ‘Unstable’ (Kodi 17.1) resolves the issue.
* Upgrading from ‘Unstable’ (Kodi 17.1) to ‘Nightly’ (Kodi 18) brings the issue back

No sure if and where older ‘nightly builds’ are available to further narrow down the issue.

Thanks!