I can confirm it is still happening on 5.3.6 RC and only for web admin mappings.

Strangely after increasing the Java heap space it started to work normally, then when I decreased it again it was still fine. What?
I decided to go outside to take a stroll.

Thanks for the hint. Mappings in the per-site web-admin are actually the configurations that we use productively. I just moved the mappings to the server administrator. So far it looks good. I'll keep an eye on it for the next days.

I will also upgrade the complete installation to the latest Lucee configuration with Tomcat 9.0.14 & OpenJDK 11.0.3 (currently Lucee 5.3.3.62 with Apache Tomcat/8.5.40 & Java 1.8.0_212 64bit).

Could you please try this:

When I map at the server-admin the mapping works

Can you confirm that server mappings are working for you too and only web mappings not?

The Lucee throw statement is here:
https://github.com/lucee/Lucee/blob/dbb220d72027fea68f927654388456515289867a/core/src/main/java/lucee/transformer/cfml/evaluator/impl/Import.java#L140

I had the same problem on a Lucee instance on CentOS 7 (with 16GB RAM) with CONTENS 5.6.2 and several smaller projects under development.

The affected Lucee versions varied between 5.2.8.50 and 5.3.3.62. The problem has been increasing for the last 1-2 months. Frequently after restarts or adjustments in templates during running operation / or corresponding deployments.

The allocation of different amounts of RAM aggravated the problem or reduced it.
Current: CATALINA_OPTS="-Xms6144m -Xmx6144m -XX:+UseG1GC -XX:MetaspaceSize=512m -XX:MaxMetaspaceSize=512m -XX:+ParallelRefProcEnabled -XX:MaxGCPauseMillis=200 -XX:InitiatingHeapOccupancyPercent=70 -XX:G1ReservePercent=15 -XX:ParallelGCThreads=20 -XX:ConcGCThreads=5 -XX:+AggressiveOpts";

But I couldn't even say that more RAM mitigated the problem.
Sometimes I got the impression that the error was forced when large applications/templates were loaded/executed too quickly shortly after Lucee restarted.

I will clarify whether SELinux has an effect on the error message.

I have just run across the same issue on a site that worked on Lucee 4.5 but we upgraded our servers and the latest stable release of Lucee 5.2.6.60 we now have this issue on one of our sites.

<cfimport taglib="/default/includes/display_objects/custom/datepicker/CustomTags" prefix="tmt">
throws an error.
invalid definition of the attribute taglib /default/includes/display_objects/custom/datepicker/Custom

I do not have a mapping set in the site Lucee admin this was working just fine without is before the upgrade. Any ideas how to fix this?